THE NEW INTERMEDIATE POLAR RX J1238-38 - A SYSTEM BELOW THE PERIOD GAP

We present optical observations of the recently discovered ROSAT sourc e RX J1238 - 38, which is a new member of the intermediate polar class of asynchronous magnetic cataclysmic variables (CVs). Optical photome try reveals two coherent periodicities at 1860 and 2147 s respectively , with similar amplitudes of similar to 8 per cent. Infrared (J-band) intensity variations are detected only at the 1860-s period, at an amp litude of similar to 15 per cent. The initial hypothesis, that these t wo periods were the spin and synodic (i.e., beat) period respectively, appears not to be supported by the spectroscopic data. The emission l ines vary on the longer photometric period, and radial velocity variat ions are detected at this period and at a longer period of similar to 5300 s, which we identify as the spin and orbital periods respectively . The most likely explanation for the 1860-s period is that it is the first harmonic of the omega - Omega sideband, leading to an improved d etermination of the orbital period as 5077 s (= 84 min). If this inter pretation is correct, RX J1238 - 38 joins EX Hya as the only other int ermediate polar below the 2-3 h period gap, and with an orbital period close to the minimum for CVs with non-degenerate secondaries. The spi n-modulated emission-line radial velocities and widths appear to be an ticorrelated, with maximum width occurring at maximum blueshift. Such an anticorrelation is expected for aspect changes of accretion curtain s. Polarimetric observations of RX J1238 - 38 were inconclusive, altho ugh we can put a limit of 0.4 per cent on any variability on the circu lar polarization, and certainly there is no indication of variations a t the photometric or spectroscopic periods.