AN OBSERVATIONAL STUDY OF THE ECLIPSING NOVA-LIKE VARIABLE BH LYNCIS (= PG0818+513)

We present photometric and spectroscopic observations of the nova-like variable BH Lyn. The continuum and line light curves exhibit deep eclipses which recur with a period of 3.74 hr. The spectra have single-peaked Balmer and He I emission lines which, due to the absorption of low-velocity emission, exhibit a double-peaked structure near the inferior conjunction of the emission-line source. The high-excitation lines of He II lambda-4686 angstrom and C III/N III lambda-lambda-4640-4650 angstrom are very weak. Radial velocity measurements show large phase shifts in all the emission lines. We construct light-centre and Doppler maps of the system which suggest that the line emission is largely non-disc in origin; for this reason we do not present a full system parameter determination. We compare and contrast BH Lyn with a number of other eclipsing nova-like variables; namely, V1315 Aql, SW Sex, DW UMa and PG 0027 + 260. The objects show basically the same behaviour, and we therefore conclude that the emission lines must be produced by a similar (unknown) mechanism. The few differences that are exhibited by BH Lyn can be attributed to the fact that the major part of our data was obtained during a temporary period of low mass transfer rate.