2 The inventory of the four categories of S Dor variables

2.1 A sub-classification based on the strength of the SD-instability

Lamers (1987) classified the individual SD-phases by means of the size of the light amplitude (note that they were called "eruptions'' by him as well as by most researchers, which I advise against). In the present paper, the S Dor variables are classified according to their maximum light amplitude reached in the 20th century. Thus, this is a classification on a centennial basis. It is quite possible that a few will enter another category in the 21st century. Some behaved differently in the 19th century. A reason to choose this kind of classification is to investigate whether the size of the instability has any influence on its present (at the turn of the 21st century) position on the theoretical HR-diagram. Our knowledge of the photometric behaviour of S Dor variables is based on a strongly varying degree of coverage of the light curve. Variables like AG Car, S Dor, $\eta$ Car, HR Car and P Cyg have been photometrically monitored for almost the whole 20th century. For others, only the last few decades were covered by photometric observations, while e.g. the HD and HDE catalogues furnish brightness data for earlier dates (Sect. 1.1). Thus, the knowledge of the photometric behaviour in the first half of the 20th century is often only fragmentary, so that some caution is appropriate. The centennial classification is defined as follows:

a)

The strong-active (s-a) members: light amplitudes >0 $.\!\!^{\rm m}$5. The cycle lengths of the SD-phases are < 10yr and 20yr (S-SD and L-SD phases, respectively). They are listed in Tables 1, 2 and 3;

b)

The weak-active (w-a) members: light-amplitudes <0 $.\!\!^{\rm m}$5. The cycle lengths of the SD-phases are < 10yr, thus they are S-SD phases. It is evident that the detection of low amplitude (say < 0 $.\!\!^{\rm m}$2) variations on a time scale of decades (representing the L-SD phases) is difficult due to too short time bases of the observations. They are listed in Tables 4, 5 and 6;

c)

The ex- and dormant (ex-/dormant) members. These objects showed no SD-type photometric variability in the 20th century as far as the scattered observations allow such a conclusion (though they do show the microvariations like all $\alpha$ Cyg variables, if photometrically well observed). Usually, they are, or have been suspected members on account of one or two of the other criteria listed in Sect. 1.3. For some specimens a strong photometric variability in the 19th century, or even earlier, has been taken as an important criterium for membership. If they would have been monitored photometrically more extensive, a few might in fact be w-a S Dor variables. They are listed in Tables 8, 9 and 10;

d)

Candidate SD variables (coined such in the literature) and former candidates (candidates rejected in the present paper). This group is divided into three subgroups, (1): the positive (+), (2): the negative (-) candidates (the first ones show a stronger SD-signature than the second ones) and (3): the former, thus, non-candidates (0). It is now certain, or at least almost certain, that the last category do not belong to the S Dor variables. They all are listed in Tables 11, 12 and 13.

It must be stressed that the stars of the four categories a) to d) show almost without any exception microvariations as described in Sect. 1.6, at least if numerous photometric data are available. Tables 14 and 15 decode the reference numbers given in Tables 1-6 and 8-13 below the header "ref.'' and mentioned in the notes below the tables, where they are bracketed.

Figure 13: The schematic light curve ($V_{\rm J}$) versus JD-2400000 of the w-a S Dor variable R85 in the LMC. Note that the magnitude HDEptg 1925 = 10.9 (Table 5) is not indicated in the figure. The broken curve represents the L-SD phase. The S-SD phase and the $\alpha$ Cyg-type microvariations are schematically sketched on scale (based on VBLUW monitoring, while the dots are in most cases individual observations by various authors), see for further details van Genderen et al. (1998b). The epochs of the three spectral type determinations are indicated by arrows and are from Feast et al. (1960), viz. around 1959, and Massey et al. (2000), viz. in 1996 and 1999 (Tables 4, 5 and 6)

Figure 16: The schematic light curve ($V_{\rm J}$) versus JD-2440000) of the w-a S Dor/B[e] variable HD34664 in the LMC. The dots are all averages of time series of VBLUW photometry and clearly represent an S-SD phase since the colours become red in the maximum (van Genderen & Sterken 1999). In order to illustrate the size of the $\alpha$ Cyg-type microvariations, a few of these time series are shown in detail, but only schematically sketched and on scale. Zickgraf et al. (1986) have given a compilation of scattered photometry, generally consisting of single observations, made between 1957 and 1984. These magnitudes $V_{\rm J}$ hover between 11.72 and 11.85 and are not shown

2.2 The selection of parameters and their accuracy

Photometric, spectroscopic and physical parameters for the same object often vary significantly from author to author. This diversity is caused by differences in assumptions and calibrations, apart from the variability of the objects. The temperature and luminosity determination of an object based on different methods may lead to substantial differences, even when applied by the same author. Uncertainties arise for example in assigned spectral types and spectral analyses and consequently in the temperature. Optical and UV spectral types sometimes disagree. Wind asymmetries might be a plausible reason (Pasquali 1997). Temperatures derived from the intrinsic colour $(B-V)_{\rm J0}$ may be unreliable due to a flatter energy distribution, the cause of the extended atmosphere. Additionally, interstellar reddening determinations are often uncertain. Further, the chosen apparent magnitudes of the extrema in the light curve, or the average apparent magnitude, to obtain M_V, as well as the applied $T_{\rm eff}$/BC calibration to find $M_{\rm bol}$, may differ from author to author. And last but not least, the error in the distance can be considerable. For galactic objects the errors in $\log T_{\rm eff}$ can easily amount to up to 0.05 ($\sim$4000K for high and $\sim$1000K for low temperatures) and 0.2 in $\log L/L_{\odot}$ (0 $.\!\!^{\rm m}$5 in $M_{\rm bol}$). For the Magellanic Cloud objects the errors in $\log T_{\rm eff}$ will be the same as above, but relative errors in $\log L/L_{\odot}$ are much smaller than 0.2, say about 0.03 for the LMC and 0.06 for the SMC objects (the difference between the two stellar systems is caused by the difference in inclination angle). The errors in the temperature mentioned above introduce errors via the BC in $\log L/L_{\odot}$, thus $\pm$ 0.1 in $\log L/L_{\odot}$ for the hottest S Dor variables, while it is negligible for the cool ones. This error should be incorporated into the error in the luminosity (for the hottest variables) given above. Apart from that, relative errors in the distance moduli between galactic and Magellanic Clouds amounting to $\sim$0 $.\!\!^{\rm m}$15 cause an extra relative error in $\log L/L_{\odot}$ of $\sim$0.06. In incidental cases the uncertainty in the distance of galactic objects is relatively larger, e.g. HD80077, Cyg OB2#12, WRA751, He3-519 and the Pistol Star. The error in $\log L/L_{\odot}$ may then amount to 0.4 ($1^{\rm m}$ in $M_{\rm bol}$). For $\eta$ Car, the M_V and the circumstellar reddening of the hidden S Dor variable are the main uncertain parameters, affecting its error box on the HR-diagram. Because of these problems, I list parameters which often are an average of various sources. Only the (hopefully) most reliable parameters were selected. References on which these data are partly or completely based, are given also. To get a more or less homogeneous set, many values from the literature, if necessary, were slightly revised by applying the relation $A_V = 3.1\,E(B-V)$ and taking $M_{V\odot}$ = 4.75. If no temperatures were known, the spectral type, or an average of literature spectra, served as a temperature indicator by applying the calibrations of de Jager & Nieuwenhuijzen (1987). The BC scale of Schmidt-Kaler (1982) was then used to derive M $_{\rm bol}$. If no spectrum was known, $(B-V)_{\rm J0}$ served as an temperature indicator. Further, all Magellanic Cloud objects have been scaled to the same distance with the distance moduli 18.45 and 19.0 for LMC and SMC, respectively. This procedure may lead in some cases to smaller scatter, yet I will use the estimated errors given above for error bars on the HR-diagram (Sect. 3).