LB-1800 - A BRIGHT ECLIPSING CATACLYSMIC VARIABLE AND A TRANSIENT-X-RAY SOURCE

The star LB 1800 has been discovered to be a 13th mag high-excitation cataclysmic variable exhibiting nova-like characteristics and partial eclipses in its light curve. It has subsequently been identified as the optical counterpart of the transient hard X-ray source 4U 0608-49 following analysis of HEAO 1 data. Both the radial velocity and photometric variations have a period of 5.56 hr, while the K-velocity of the relatively uncomplicated radial velocity curves is 134 ± 9 km s-1, leading to a mass function of 5.8 ± 0.3 × 10-2 M⊙. The dynamical and eclipse solutions point to an orbital inclination of 87° ± 3° and a mass ratio of 0.46 ± 0.04 for the empirical secondary mass 0.55 M⊙. The implied white dwarf mass is therefore quite high at ∼ 1.2 M⊙. We estimate the disk size, from timings of first and last contact, to be very close to that of the secondary star. Phase-dependent, double-peak emission lines are sometimes observed. We argue that the orbital variations in the relative strengths of the red and blue peaks arise from changing hot-spot visibility. Also we contend that the double peaks may be a result of an absorption self-reversal. Such an interpretation is supported by the observed wavelength dependence of this reversal in the Balmer lines, and we note that the maximum depth of this central reversal, and the maximum strength of Ca H II and K absorption, occurs at the orbital phase (∼ 0.8-0.9) when the cool accretion stream is seen projected against the bright disk. In many respects LB 1800 resembles other high-excitation eclipsing nova-like systems (e.g., SW Sex, V1315 Aq1, and DW UMa), although it appears to be more easily understood in terms of the canonical model for a cataclysmic variable.