3 On the short-term light curves

Once the long-period sinusoidal modulation is subtracted, the phase curve of the residuals (Fig. 1) turned out to be of three types: eclipsing, double-wave and single-wave. The light curve of the eclipsing stars are typical of those found in Algol type variables. On the other hand, the fact that the stars showing double-wave or single-wave light curve share similar long-term variability with the eclipsing stars and exhibit similar correlation between the short-term and long-term period (see next section) suggests that we are in presence of an homogenous group of short-period variables, and that these non-eclipsing stars could also be Algol-like stars but seen under higher orbital inclinations. The period range along with the fact that emission lines have been observed in type-3 stars (M02 and Keller et al. 2002) are consistent with this interpretation. In this view, double-wave light curves could arise from the changing aspects of a non-spherical Roche-lobe filling secondary star, as occurs in the suspected Algol-type binary V 1080 (Simon et al. 2000). It is worth to mention that long-term periodic oscillations have been reported in a few Algol systems but with very low amplitude (e.g. Walter 1981). The fact that Algols contain B-A type primaries and cooler secondaries should explain the distribution of type-3 stars in their colour-colour diagram (M02). In 5 cases we found single-wave light curves. It is possible that the true period is twice the reported period in these cases. This is confirmed by the similar power shown by P₁ and $2\times P_{1}$in the periodogram and also by the good fit of  $2\times P_{1}$ by the P₁-P₂ linear correlation (see next section).

Figure 1: Eclipsing (upper graph) and double-wave (below graph) light curves. The three pannels in every graph show, from bottom to top, the long-term light variability, the residuals from a sinusoidal fitting and the residuals folded using the short-term period.

Figure 2: MACHO b and b-r light curves folded with the long-term period.