A 421-d activity cycle in the BeX recurrent transient A0538-66 from MACHO monitoring

ABSTRACT We present a ∼5-yr optical light curve of the recurrent Be/X-ray transient A0538–66obtained as a by-product of the MACHO Project. These data reveal both a long-termmodulation at P = 420.8±0.8 d and a short-term modulation at 16.6510±0.0022 dwhich, within errors, confirms the previously found orbital period. Furthermore, theorbital activity is only seen at certain phases of the 421 d cycle suggesting that thelong-term modulation is related to variations in the Be star envelope.Key words: binaries: close - stars: individual: A0538–66 - X-rays: stars 1 INTRODUCTIONThe recurrent X-ray transient A0538–66 was discovered withAriel V when two outbursts, separated by ∼17 d, were ob-served (White and Carpenter 1978). Further outbursts wereobserved with HEAO-1 which, when the source was active,was found to have a periodicity of 16.668 d (Johnston etal. 1979; Johnston et al. 1980; Skinner et al. 1980; Skinner ⋆ email: kem@astro.ox.ac.uk 1980), the precision of which led to its interpretation as be-ing orbital. Skinner (1981) used archival plates taken over∼50 years in order to obtain an improved value of 16.6515 dfor this periodicity, based on the recurrence of the outbursts.A0538–66 has been optically identified with a B star(V∼15) (Johnston et al. 1980), and an X-ray pulse periodof 69 ms indicates that the compact object is a neutronstar (Skinner et al. 1982). In quiescence (X-ray ’off’) thecolour, magnitude and radial velocity are consistent with aB2 III-IV star in the LMC (Charles et al. 1983), but during