DR Tau: The 2.5 million-year-old object DR Tau belongs to the Taurus-Auriga star-forming region at a distance of 140 pc (Siess et al. 1999). Numerous publications illustrate that DR Tau is one of the most well-studied T Tauri objects. Strong veiling has been found in the visual and NIR wavelength range: the flux in V band, for instance, exceeds the intrinsic stellar flux by a factor of five (Edwards et al. 1993). Apart from the excessive flux in the visual, the profiles of several emission lines, such as the Pf-line, points to material accreting onto the central star while the profiles of further emission lines such as the He I line (Kwan et al. 2007) and the H lines (Vink et al. 2005) are evidence of outflowing stellar/disk winds. Based on a modeling study, Edwards et al. (1993) determined an accretion rate of , while a mass of is lost by stellar/disk winds. A strongly collimated outflow was also found by Kwan & Tademaru (1988). DR Tau is photometrically and spectrally variable on short terms, i.e., in the range of weeks ( mag; Grankin et al. 2007; Eiroa et al. 2002; Smith et al. 1999). This variability is also ascribed to the formation and movement of stellar spots on this star (Ultchin et al. 1997).
GW Ori: GW Ori, also known as HD 244138, belongs to the star-forming region B 30 in a ring-shaped molecular cloud close to Ori. According to Dolan & Mathieu (2001), the ring shape has its origin in a central supernova explosion 1 million years, ago. As GW Ori is 1 million years old (Mathieu et al. 1991), the supernova explosion could cause the formation of this object. Considering a stellar luminosity of several tens of solar luminosities (Mathieu et al. 1997; Calvet et al. 2004), GW Ori is one of the most luminous YSOs with a spectral type of G0. Using theoretical evolutionary tracks, a stellar mass of 2.5 (Mathieu et al. 1991) and 3.7 (Calvet et al. 2004) could be derived. GW Ori is a spectroscopic binary (Mathieu et al. 1991). The companion with a mass of 0.5 up to 1 orbits the primary at a projected distance of 1 AU in 242 days. Mathieu et al. (1997) used two different modeling approaches to reproduce the SED of this system. In their first approach, the secondary creates a (gas and dust free) gap between 0.17 AU and 3.3 AU. Their second modeling approach, where the circumbinary disk was replaced by a spherical envelope, disagreed with subsequent millimeter measurements at the James Clerk Maxwell Telescope (Mathieu et al. 1995). A disk mass of 0.3 could be derived using the latter set of millimeter measurements, where the object could be spatially resolved. An outer radius of 500 AU and an inclination angle of were determined. Artymowicz & Lubow (1994) showed that the disk gap of GW Ori proposed by Mathieu et al. (1991) cannot be explained by a theoretical modeling study of tidal forces. Calvet et al. (2004) measured an accretion rate of , but the source does not reveal any veiling in the visual. To stabilize a high accretion rate for several 100 000 years, Gullbring et al. (2000) proposed the existence of an additional massive envelope where an inner cavity enables the observation of the inner disk edge. GW Ori is only weakly variable ( mag; Grankin et al. 2007). Mathieu et al. (1991) pointed to a second companion with a period of 1000 days that was found by the movement of the center of gravity in the system.
HD 72106 B: The object HD 72106 is a visual binary (angular distance , ) in the Gum nebula at a distance of (Torres et al. 1995; Hartkopf et al. 1996; Fabricius & Makarov 2000). The H-emission as well as the infrared excess are ascribed to the visually fainter B-component ( ), while the A component already belongs to the main sequence (Vieira et al. 2003; Wade et al. 2005) and shows a strong magnetic field that was found with spectropolarimetry (Wade et al. 2005; Wade et al. 2007). The faint H emission line as well as the broad silicate emission band that can be compared with the silicate band of the comets Hale-Bopp and Halley point to the advanced evolutionary status of the B component as a YSO (Vieira et al. 2003; Schütz et al. 2005). In particular, Schütz et al. (2005) found larger amounts of enstatite () that effectively contributes to the silicate band. This large spectral contribution of enstatite has only been found for the evolved Herbig Ae/Be objects, HD 100546 and HD 179218, so far. Folsom (2007) intensively studied this binary system.
RU Lup: RU Lup is a classical T Tauri star in the star-forming region Lupus. Visual and millimeter measurements showed that the object does not have a (remaining) circumstellar envelope (Giovannelli et al. 1995; Lommen et al. 2007). A secondary could not be found with speckle interferometry down to a minimal distance of in the NIR range (Ghez et al. 1997) and with the Hubble Space Telescope (Bernacca et al. 1998). Broad visual emission lines as well as flux variations in the U and V band are evidence of accreting material. Lamzin et al. (1996) determined a mass accretion rate of . The stellar magnetic field of the object has a strength of (Stempels et al. 2007). Absorption lines shifted to longer wavelengths results from an outflowing stellar wind (Herczeg et al. 2005).
HBC 639: HBC 639, also known as DoAr 24 E, belongs to the star-forming region Ophiuchi. It is a class II object (McCabe et al. 2006) with an age of between 1 and 3 million years (Gras-Velázquez & Ray 2005). Because the H line reveals an equivalent width of 5 Å, HBC 639 is a weak-line T Tauri object, where the circumstellar material has evolved more rapidly than in classical T Tauri stars of similar age. However, the infrared excess points to a remaining circumstellar disk in the system. The accretion rate inferred from the width of the Pa and Br lines is low: (Natta et al. 2006). HBC 639 has an infrared companion at an angular distance of ( for ) and a position angle of measured from North to East (Reipurth & Zinnecker 1993). The brightness of the secondary increases in the infrared and exceeds the brightness of the primary already in L band (Prato et al. 2003). Chelli et al. (1988) assumed that the primary effectively contributes only to the NIR and MIR flux of the system. Polarimetric measurements in the K band showed that both components have a circumstellar disk with almost identical position angles ( ; Jensen et al. 2004). The companion is a class I object and active (Prato et al. 2003). Using the speckle-interferometric technique in K band, Koresko (2002) found a second companion close to the secondary. Both latter companions have a similar brightness in K band.
S CrA: The source S CrA belongs to the Southern Region of the Corona Australis Complex (e.g., Chini et al. 2003). The source was already defined as a T Tauri object by Joy et al. (1945) who pointed to an infrared companion at a projected distance of and at a position angle of (Joy & Biesbrock 1944; Reipurth & Zinnecker 1993). Highly spatially resolved observations in the NIR wavelength range showed that both objects have an active circumstellar disk (Prato et al. 2003). The spectral lines of both objects indicate that they have similar shapes and depths. Both components are probably coeval (Takami et al. 2003). S CrA is a YY Ori object, i.e., the emission lines are asymmetric and shifted to longer wavelengths. These lines arise from material that accretes onto the central star. The measured spectral variability is another hint of non-continuous accretion process. Prato & Simon (1997) showed that only an infalling circumbinary envelope provides enough material for accretion in the long-term.
Table A.1: Photometric flux measurements of DR Tau.
Table A.2: Photometric flux measurements of GW Ori. The value that is marked with the symbol `` '' is an upper flux limit.
Table A.3: Photometric flux measurements of HD 72106 B. The value that is marked with the symbol `` '' is an upper limit.
Table A.4: Photometric flux measurements of RU Lup.
Table A.5: Photometric flux measurement of HBC 639. The flux can be ascribed to the main component up to wavelengths in L band.
Table A.6: Photometric flux measurement of S CrA N and S CrA S. For the photometric observations of wavelengths and , both components could not be spatially resolved.