Periodicities in the high-mass X-ray binary system RX J0146.9+6121/LS I+61°235

The high-mass X-ray binary RXJ0146.9+6121, with optical counterpart LS I+61 degrees 235 (V831 Cas), is an intriguing system on the outskirts of the open cluster NGC 663. It contains the slowest Be type X-ray pulsar known with a pulse period of around 1400 s and, primarily from the study of variation in the emission line profile of Ha, it is known to have a Be decretion disc with a one-armed density wave period of approximately 1240 d. Here we present the results of an extensive photometric campaign, supplemented with optical spectroscopy, aimed at measuring short time-scale periodicities. We find three significant periodicities in the photometric data at, in order of statistical significance, 0.34, 0.67 and 0.10 d. We give arguments to support the interpretation that the 0.34 and 0.10 d periods could be due to stellar oscillations of the B-type primary star and that the 0.67 d period is the spin period of the Be star with a spin axis inclination of 23(-8)(+10) degrees. We measured a systemic velocity of -37.0 +/- 4.3 km s(-1) confirming that LS I+61 degrees 235 has a high probability of membership in the young cluster NGC 663 from which the system's age can be estimated as 20-25 Myr. From archival RXTE All Sky Monitor (ASM) data we further find 'super' X-ray outbursts roughly every 450 d. If these super outbursts are caused by the alignment of the compact star with the one-armed decretion disc enhancement, then the orbital period is approximately 330 d.