X-ray and optical periodicities in X-ray binaries - I. A0535+26

A homogeneous set of UBV photometry (354 data points obtained between 1983 and 1998) for the Be/X-ray binary A0535+26 = V725 Tau is analysed, aiming t o look for possible periodic component(s). After subtraction of the long-te rm variation it was found that only a approximate to 103(d) periodic compon ent remains in the power spectra in both the V and B colour bands. The prob ability of chance occurrence of such a peak is less than 0.1%. There are no signs of optical variability at the X-ray period (approximate to 111(d)). We discuss possible reasons for a 103-day modulation and suggest that it co rresponds to a beat frequency of the orbital period of the neutron star and the precession period (approximate to 1400(d)) either of an accretion disc around the neutron star or a warped decretion disc around the Be star.