1 Introduction

Over the past years, the microlensing projects (OGLE, MACHO, EROS) have monitored millions of stars in the Magellanic Clouds and Galactic bulge for variability. The resulting huge photometric databases are very well suited not only for microlensing studies but also for many other issues of modern astrophysics, including the distance scale, variable stars, star clusters etc. In particular, the OGLE-II project (Udalski et al. 1997), has provided accurate BVI measurements for about 6.5 million stars from the central parts of the Magellanic Clouds (Udalski et al. 1998, 2000). Based on this same material, a unique catalog containing about 68 000 variable stars has just been released (Zebrun et al. 2001).

One example of the scientific information that it is possible to extract from such a catalogue is shown by the work of Mennickent et al. (2002, hereafter M02), who report the discovery of Be star candidates in the Small Magellanic Cloud showing unexpected photometric variations. Basically, these authors found four types of variability in targets within the luminosity-colour box of typical Be stars: type-1 stars showing outbursts, type-2 stars showing sudden magnitude jumps, type-3 stars showing periodic variations and type-4 stars showing random variability. According to these authors, possible causes for type-1 and type-2 stars include blue pre-main sequence stars surrounded by thermally unstable accretion discs and white dwarfs accreting from the Be star envelope in a Be+WD binary. Type-4 stars could be classical Be stars. On the other hand, type-3 stars could not be linked to the Be star phenomenon at all, mainly due to their rather red colours and strictly periodic behaviour. The above study is complemented by the work by Keller et al. (2002) on blue variable stars from the MACHO database in the LMC. These authors report basically the same photometric behaviour in a sample of LMC blue variables. These authors also find emission lines in the spectra of many of these variables.

Further inspection of the mysterious type-3 stars found in the SMC and the LMC (Mennickent et al. in preparation) revealed 30 objects showing double-periodic photometric variations: a long-period sinusoidal variation and a short-period modulation. Some of these short-term light curves are typical of eclipsing Algol variables. The study of this new double-period photometric variability is the subject of this paper.