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Appendix A: Notes on individual systems

A.1. Eclipsing sdB+MS/BD PCEBs

HW Vir is the prototype of HW Vir-like systems consisting of an sdB star with a late-type MS or BD companion. HW Vir systems have short periods (~2−3 h), and the light curves show very sharp primary and secondary minima and a strong reflection effect. HW Vir itself was discovered by Menzies & Marang (1986), who determined the period. Kilkenny et al. (1994) observed a change in the orbital period for the first time, which motivated further studies (e.g., Çakirli & Devlen 1999; Wood & Saffer 1999; Kiss et al. 2000; Kilkenny et al. 2000, 2003; İbanoǧlu et al. 2004). Many possible explanations have been proposed and discussed, converging towards the existence of a third object with a long period and low mass. Qian et al. (2008) present new mid-eclipse times obtained from 2006 to 2008 that show some deviation from the sinusoidal fit proposed by Kilkenny et al. (2003) and İbanoǧlu et al. (2004). They suggest a combination of cyclic variations plus a continuous decrease in the orbital period, which may reveal a fourth object with a long period. Lee et al. (2009) present 41 new mid-eclipse times taken from 2000 to 2008 and combine these with data from the literature. The Observed minus Calculated (O−C) diagram of the orbital period spanning more than 24 years shows a combination of two sinusoidal variations, probably produced by the presence of two substellar companions, plus a continuous period decrease that is too strong to be caused by gravitational radiation. Recently, Beuermann et al. (2012b) have published 26 new mid-eclipse times obtained between 2008 February and 2012 February, which deviate significantly from the Lee et al. (2009) prediction. They also find that the solution presented by Lee et al. (2009) is unstable and propose a new solution involving two companions to HW Vir: a planet and a BD or low-mass star.

HS 0705+6700 is an sdB+dM binary discovered to be a detached short-period eclipsing system by Drechsel et al. (2001). Qian et al. (2009b) obtained 38 mid-eclipse times between 2006 and 2008, updated the ephemeris, and performed an O−C diagram including 31 mid-eclipse times obtained from the literature since 2000 October (Drechsel et al. 2001; Niarchos et al. 2003; Németh et al. 2005; Kruspe et al. 2007). They detected cyclic variations that were attributed to the light-travel time effect produced by the presence of a third object. Qian et al. (2010c) present new mid-eclipse times taken in 2009, propose the existence of a continuous decrease in the orbital period, and derive a mass for the third body corresponding to a substellar object, probably a BD. Çamurdan et al. (2012) obtained new mid-eclipse times in 2010 December, which disagree with a long-term period decrease. They used a sinusoidal fit to adjust a third object in the system, which may be substellar or a very low-mass star, depending on the inclination. Recently, Beuermann et al. (2012a) have published new mid-eclipse times obtained between 2009 August and 2011 December, updated the ephemeris, and derive the parameters for a possible third body that seem to be more consistent with a substellar object.

HS 2231+2441 was discovered to be an eclipsing sdB with a low-mass, probably substellar, companion by Østensen et al. (2007), who determined the ephemeris and estimated the masses. Mid-eclipse times monitored since 2006 (Qian et al. 2010c) reveal a continuous decrease and cyclic variations in the orbital period. The cyclic oscillation suggests there is a third object in the system, and Qian et al. (2010c) estimate it may be a BD.

NSVS 14256825 was found to be an eclipsing sdB+dM binary by Wils et al. (2007), who presented 19 primary mid-eclipse times obtained between 2007 June and September and derived the ephemeris and orbital parameters, which were recently updated by Almeida et al. (2012b). The system has also been monitored since 2006 by Qian et al. (2010c), whose preliminary results suggest a cyclic change in the orbital period. New mid-eclipse times obtained between 2010 September and 2011 October were published by Kilkenny & Koen (2012). They found an increase in period, but a long baseline is needed to see whether this behavior is cyclic. In a parallel work, Beuermann et al. (2012a) present new mid-eclipse times obtained between 2009 July and 2011 October, which reveal an abrupt and continuous increase in the period after 2009. This is interpreted by the authors as the possible response to a third body in a highly elliptic orbit. In a very recent work, Almeida et al. (2012a) have reanalyzed the system including ten new mid-eclipse times between 2010 July and 2012 August and find that the variations observed in the O−C diagram can be explained by two circumbinary giant planets.

NY Vir is an sdB+dM binary whose eclipsing nature was revealed by Kilkenny et al. (1998). Subsequently, Kilkenny et al. (2000) present mid-eclipse times from 1996 to 1999, determined the ephemeris, and find no significant changes in the orbital period. However, Kilkenny (2011) combine the previous results with new mid-eclipse times taken between 2001 and 2010 and find a continuous period decrease, which is too high to be due to gravitational radiation. The strong decrease was also seen by Çamurdan et al. (2012), who have presented new mid-eclipse times from 2009 and 2011 and adjusted a downward parabola to the O−C diagram. Recently, Qian et al. (2012b) have combined the previous results with nine new mid-eclipse times obtained in 2011 May, updated the ephemeris, and propose that the O−C diagram can be adjusted by a downward parabola plus a periodic variation produced by a planet orbiting the primary. They also suggest that the continuous period decrease may be part of a cyclic variation that may indicate the presence of a fourth object.

2M1938+4603 was found to be an eclipsing sdB+dM binary by Østensen et al. (2010), who derived the ephemeris and orbital parameters. The available mid-eclipse times only cover the period between 2008 June and 2010 May and therefore no conclusion about apparent period variations can be drawn yet.

NSVS 07826247 is the longest period sdB+dM binary known to be eclipsing so far, discovered by Kelley & Shaw (2007). Mid-eclipse times have been published by For et al. (2010, 2008 February to 2009 March), Liying & Shengbang (2010, 2009 March to August), and Backhaus et al. (2012, 2011 February to October). No evidence of period variation has been found so far, but more observations are needed in order to discard variations.

BUL-SC16 335 was found to be an eclipsing system by Polubek et al. (2007), who suggest that it might be an HW Vir-like system based on the appearance of the light curve. They derived the ephemeris and estimate some of the orbital parameters. Tello & Jablonski (2010) rederived the orbital parameters and find disagreement with Polubek et al. (2007). The parameters are still poorly constrained and no mid-eclipse times are available so far.

PG 1621+4737 has been found in the course of the MUCHFUSS (massive unseen companions to hot faint underluminous stars from SDSS, Geier et al. 2011a) project by Geier et al. (2010), who determined the period and estimated that the companion to the sdB star is a very late-type MS star or a BD. The system parameters are not constrained and there are still no published mid-eclipse times.

SDSS J0820+0008 was discovered by Schaffenroth et al. (2011), who detected the short period radial velocity variations, observed the eclipses in the light curve indicating an HW Vir-like systems, and determined the period. The mass of the sdB primary covers a wide range of possible solutions, and the companion seems to be a BD. The ephemeris was derived by Geier et al. (2011b). Based on high-resolution spectroscopy with ESO-VLT/UVES Geier et al. (2012) confirm the substellar nature of the companion. They also find a significant shift in the system velocity with respect to the previous study, which may be produced by a third object in the system. However, more observations are needed.

ASAS 10232 was discovered by Schaffenroth et al. (2011), who determined the period and observed the primary eclipse in the light curve. The reflection effect, typical of HW Vir-like systems, is clearly observed. The orbital parameters were derived based on photometry and spectroscopy, and no mid-eclipse time has been published so far.

AA Dor is a short-period eclipsing binary containing an sdOB primary (Kilkenny et al. 1978). Although many subsequent investigations have been published (e.g., Kilkenny et al. 1979, 1981; Kudritzki et al. 1982; Rauch 2000; Hilditch et al. 2003; Fleig et al. 2008; Vučković et al. 2008; Rucinski 2009; Müller et al. 2010; Klepp & Rauch 2011), the nature of the companion is still not clear, with some authors favoring a BD companion while others favor a very low-mass M star. Eclipse timings were studied by Kilkenny et al. (2000), who found no evidence of any period variations between 1977 and 1999. Kilkenny (2011) increased the baseline by including 13 new primary eclipse timings obtained between 2000 and 2010, updated the ephemeris, and confirmed the stability of the orbital period.

EC 10246-2707 has recently been reported as an eclipsing sdB+dM binary by Barlow et al. (2012), who estimate the orbital parameters, derive the ephemeris, and present 49 mid-eclipse times covering 15 years between 1997 February and 2012 June. The O−C diagram reveals no secular changes larger than 10^-12 s s^-1 in the period. However, the relatively low precision of the timings does not allow to rule out small-amplitude variations such as, e.g., those observed in NN Ser. Additional observations of eclipses with high precision are needed.

A.2. Eclipsing WD+MS PCEBs

NN Ser was discovered to have deep eclipses by Haefner (1989), who classified the system as a DA+dM binary and measured the orbital period. Brinkworth et al. (2006) performed high-time-resolution photometry with ULTRACAM and detected a decrease in the orbital period that Applegate’s mechanism fails to explain. They suggest it may be related to the presence of a third body. Qian et al. (2009a) propose a sinusoidal fit to the O−C diagram. Later, Parsons et al. (2010a,b) presented the result of eclipse observations performed with ULTRACAM since 2002, which disagree with the sinusoidal fit proposed by Qian et al. (2009a). However, they still consider that a third body may be the cause of the observed period changes. Beuermann et al. (2010) monitored the system during 2010 and combined their new mid-eclipse times with all the previously published times, reanalyzing those where it was necessary. They conclude that the large amplitude variations observed in the period can only be caused by a third body, and even suggest that the best model is obtained with two giant planets around the binary. However, the existence of the fourth body is still rather uncertain.

V471 Tau is a DA+dK2 system discovered to be eclipsing by Nelson & Young (1970). Period variations have been observed for a long time (e.g., Lohsen 1974), and many possibilities were suggested in the past to explain the shape of the O−C diagram (e.g., Ibanoglu et al. 1994; İbanoǧlu et al. 2005; Guinan & Ribas 2001; Kamiński et al. 2007), such as perturbations by a third body in a long-period orbit, apsidal motion due to a low orbital eccentricity, or even mass transfer. Recently, Kundra & Hric (2011) have detected a change in the O−C diagram trend. This allows them to exclude mass transfer and other models that predict a further increase in the O−C value. After modeling the system, the authors find that the third component could be a BD or a very low-mass star, at a period of 33.2 years.

QS Vir was discovered in the Edinburgh-Cape faint blue object survey of high galactic latitudes (Kilkenny et al. 1997), where the eclipses revealed its binary nature. O’Donoghue et al. (2003) suggest that it is a hibernating CV, which was questioned by Ribeiro et al. (2010) and ruled out by Parsons et al. (2011b), who confirm that it is a detached system and not a hibernating CV based on high-resolution UVES spectra. Orbital period variations were analyzed by Qian et al. (2010b), who combined new and previously published mid-eclipse times, and propose that there is a giant planet and a continuous decrease in period due to magnetic braking. Parsons et al. (2010b) update the O−C diagram by including ULTRACAM photometry and find strong disagreement with the fit of Qian et al. (2010b). They conclude that the decrease in orbital period is part of a cyclic variation that cannot be explained by Applegate’s mechanism. A third body seems to be the most probable solution. Recently, Almeida & Jablonski (2011) have presented new mid-eclipse times and suggest that the best fit in the O−C diagram is obtained with a model with two circumbinary bodies, most likely a giant planet and a BD.

RR Cae is a WD+dM binary discovered as a high proper motion object by Luyten (1955). The eclipses were first announced by Krzeminski (1984), and further observations of the eclipses were presented by Bruch & Diaz (1998) and Maxted et al. (2007). The later paper updated the ephemeris and finds no evidence of variations in the orbital period on a long time scale (~10 yr). Parsons et al. (2010b) performed ULTRACAM photometry for the system, obtained two new mid-eclipse times, and combined these with all the previous eclipse times available in order to study possible period variation. They obtained a roughly sinusoidal variation in the O−C diagram, which can be explained via Applegate’s mechanism. Qian et al. (2012a) have recently obtained six new mid-eclipse times that combined with those from the literature, show some evidence of a third object, a giant planet, and even possible evidence of a fourth companion. The last needs to be confirmed.

DE CVn is a bright eclipsing WD+dM binary discovered as an X-ray source by ROSAT (Voges et al. 1999). Recently, Parsons et al. (2010b) have obtained high-time-resolution photometry with ULTRACAM to obtain an accurate ephemeris. They combined their new mid-eclipse times with older times available in the literature (Robb & Greimel 1997; van den Besselaar et al. 2007; Tas et al. 2004) in order to study possible period variations. However, only the ULTRACAM data are reliable, and there are still too few to analyze possible long-term period changes.

GK Vir was discovered by Green et al. (1978), who listed nine mid-eclipse times from 1975 April to 1978 February, and determined the orbital period. For more than twenty years there were no new eclipses observed. Between 2002 and 2007 Parsons et al. (2010b) observed seven primary eclipses with ULTRACAM and improved the ephemeris. After combining their points with those from Green et al. (1978), they observed a period increase and a slight variation in O−C times. Drake et al. (2010) also published one mid-eclipse time for 2005 April. Recently, Parsons et al. (2012b) have obtained a new high-precision mid-eclipse time on April 2010 using ULTRACAM, which shows a clear deviation from linearity. The magnitude of the period change is small and therefore can be caused by Applegate’s mechanism or due to a third body in the system. More data is needed before the true cause of this period change can be established.

RX J2130.6+4710 was discovered to have eclipses by Maxted et al. (2004), who determined the ephemeris and published three ULTRACAM mid-eclipse times obtained in 2002 and 2003. No new mid-eclipse times are available.

SDSS J0110+1326 and SDSS J0303+0054 were identified as eclipsing binaries by Pyrzas et al. (2009), who studied the eclipses between 2006 September and 2007 October, derived the ephemeris, and listed four and seven mid-eclipse times, respectively. Subsequently, Parsons et al. (2010b) analyzed accurate eclipses obtained during 2007 October using ULTRACAM and listed one and three new mid-eclipse times, respectively. They found some deviations from the ephemeris derived by Pyrzas et al. (2009) for SDSS J0110+1326, while the eclipses for SDSS J0303+0054 appear to be consistent in the two studies. However, given the short baseline and the large uncertainty in the mid-eclipse times derived by Pyrzas et al. (2009) further accurate observations are needed before long-term period changes can be explored. Recently, Backhaus et al. (2012) have presented six new mid-eclipse times for SDSS J0303+0054 between 2011 August and November, which increases the baseline for this system. The new eclipses are still consistent with linear ephemeris, however the authors still do not exclude long-term period variations.

SDSS J0857+0342 was first listed as an eclipsing system by Drake et al. (2010), who observed the regular eclipses as part of the Catalina Realtime Transient Survey and determined the short orbital period, which makes it the closest detached WD+dM binary. They published a mid-eclipse time for 2005 April. Recently, Parsons et al. (2011a) have presented nine new mid-eclipse times obtained between 2010 December and 2011 January with ULTRACAM, and updated the ephemeris that is so far consistent with the eclipse time listed in Drake et al. (2010). Independently, Backhaus et al. (2012) obtained seven more mid-eclipse times between 2010 November and 2011 October. Due to the uncertainty in Drake’s eclipse, possible period changes cannot be discarded so far.

SDSS J1210+3347 was discovered to be eclipsing by Pyrzas et al. (2012), who obtained high-time-resolution photometry for nine eclipses between 2009 April and 2011 June using RISE on the Liverpool Telescope. They determined the orbital period and the ephemeris, however the mid-eclipse times still cover a short period of observation to study changes in the period.

SDSS J1212-0123 was listed as an eclipsing WD+dM binary by Nebot Gómez-Morán et al. (2009), who obtained six mid-eclipse times between 2007 January and 2008 May and derived the ephemeris. Parsons et al. (2012b) used ULTRACAM to obtained the first high-precision mid-eclipse in 2010 April, which was consistent with the mid-eclipse times from Nebot Gómez-Morán et al. (2009). They updated the ephemeris using the new accurate eclipse.

SDSS J1435+3733 is a partially eclipsing binary discovered by Steinfadt et al. (2008), who observed three eclipses in 2007 May and June. In a parallel study Pyrzas et al. (2009) independently identified the system, observed seven eclipses between 2007 February and May, and updated the ephemeris.

SDSS J1548+4057 was found to be eclipsing by Pyrzas et al. (2009), who derived the ephemeris and listed seven mid-eclipse times from 2008 May to July. Recently, Backhaus et al. (2012) presented six new mid-eclipse times between 2011 May and August, which are consistent with linear ephemeris but are still not enough to exclude long-term period variations.

CSS06653, CSS080502, CSS21616, CSS38094, CSS40190, CSS41631, and WD 1333+005 are all WD+dM binaries discovered to be eclipsing by Drake et al. (2010) as part of the Catalina Realtime Transient Survey. They only list one mid-eclipse time in 2005 April or May. Recently, Backhaus et al. (2012) present new mid-eclipse times obtained between 2011 January and October for all these systems, and derived the ephemeris again. The accuracy of Drake’s eclipses is not good enough to confirm or exclude long-term period variations. However, most of the systems seem to be consistent with linear ephemeris (within a 2-σ error), except for CSS06653 and WD 1333+005, which exhibit an increase in the observed period that is more than the expected error with respect to Drake’s mid-eclipse times. More observations are necessary to confirm or discard these trends.

CSS07125, CSS080408, CSS09704, CSS09797, CSS21357, and CSS25601 were also discovered to be eclipsing WD+dM binaries by Drake et al. (2010) with one mid-eclipse time in 2005 April or May listed, except in the case of CSS21357 where no eclipse time is presented. No further studies are available for these systems.

PTFEB11.441, PTFEB28.235 and PTFEB28.852 are three eclipsing systems recently discovered by Law et al. (2012) during the PTF/M-dwarf survey (Law et al. 2011) for transiting planets around M-dwarfs. Only one mid-eclipse time is available for each system in 2010 August, September, and November, respectively.

KIC-10544976 is a WD+MS PCEB in the field of Kepler mission, recently published by Almenara et al. (2012). The authors list then mid-eclipse times: six in 2005, three in 2006, and one in 2008. Currently, the system has been continuously monitored by Kepler over two years, which makes it a good candidate to look for changes in the period in the near future.

SDSS J0821+4559, SDSS J0927+3329, SDSS J0946+2030, SDSS J0957+3001, SDSS J1021+1744, SDSS J1028+0931, SDSS J1057+1307, SDSS J1223-0056, SDSS J1307+2156, SDSS J1408+2950, SDSS J1411+1028 and SDSS J2235+1428 are all WD+MS PCEBs that were found to be eclipsing very recently by Parsons et al. (2012a) by correlating the SDSS WD+MS catalog (Rebassa-Mansergas et al. 2012) with Catalina Real-time Transient Survey light curves. Only one mid-eclipse time is available for each system.

A.3. CVs with suspected planets

UZ For is an eclipsing polar (or AM Her type CV) discovered by Beuermann et al. (1988). The orbital period has been analyzed by several authors (e.g., Ramsay 1994; Imamura & Steiman-Cameron 1998; Perryman et al. 2001), who detected variations from linearity in the O−C diagram. Dai et al. (2010) detected an increase in the orbital period and possible cyclical changes, which they suggest may be produced by a third low-mass object in the system. Recently, Potter et al. (2011) have combined new high-speed photometry spanning ten years with previous mid-eclipse times to obtain a baseline of 27 years. They find that the O−C diagram is described best by a combination of two cyclic elliptical terms, probably due to the presence of two giant planets, and a secular variation.

HU Aqr is also an eclipsing polar discovered by Schwope et al. (1993) during the ROSAT All Sky Survey. Schwarz et al. (2009) updated the ephemeris and listed 72 eclipse egress times obtained between 1993 and 2007, revealing complex deviations from a linear trend in an O−C diagram. Qian et al. (2011) combined these points with ten new eclipse egress times obtained between 2009 May and 2010 May, and find two cyclic variations in the O−C curve and a long-term period decrease that cannot be explained by gravitational radiation or Applegate’s mechanism. They propose that the cyclic variations observed are due to the presence of two giant planets, and the long-term period decrease may reveal a third planet. However, the proposed orbital parameters for the two possible planets have been questioned by Horner et al. (2011), who find the solutions extremely unstable on short time scales. Wittenmyer et al. (2011) used the same 82 eclipse egress times to fit different single- and double-planet models to the O−C diagram. They found that the best fits are obtained with two planets and that there is no need to invoke a third planet. Although their solutions are significantly different from the one given by Qian et al. (2011), the new parameters for the possible planets are still dynamically unstable, which casts some doubt

on their existence. They speculate about other mechanisms being responsible for the observed variations, such as changes in the shape of the secondary star due to dynamo effect. However, in a very recent work, Goździewski et al. (2012) publish almost 60 new eclipse egress times with better accuracy. Combining these with reanalyzed previous data, the authors find that a single circumbinary companion gives the best explanation for the O−C curve.

DP Leo was the first eclipsing polar discovered (Biermann et al. 1985). A decrease in the binary period was noticed by Schwope et al. (2002) and Pandel et al. (2002). Qian et al. (2010a) find a reversal of this trend, suggesting a sinusoidal variation that may be related to the presence of a giant planet. Beuermann et al. (2011) obtained accurate mid-eclipse times of the WD between 2009 March and 2010 February, updated the ephemeris, and combined their results with all the mid-eclipse times available by 2002, as published by Schwope et al. (2002). The data spanning more than 30 years since 1979 suggest there is a third body orbiting the binary, most likely a giant planet.

© ESO, 2013