Long-term optical and X-ray observations of the old novae DI Lacertae and V841 Ophiuchi

We present an analysis of ground-based optical photometry and spectroscopy and Rossi X-Ray Timing Explorer X-ray observations of the old novae DI Lace rtae and V841 Ophiuchi. Our optical photometry data (obtained with the auto mated photometry telescope RoboScope) comprise an almost decade-long light curve for each star, while the contemporaneous spectroscopy and X-ray obser vations repeatedly sampled each nova during separate intervals of approxima te to 45-55 days in length. The long-term optical light curves of both nova e reveal quasi-periodic variability on typical timescales of similar to 30- 50 days with amplitudes of DeltaV similar to0.4-0.8 mag. V841 Oph also disp lays a long-term, sinusoidal modulation of its optical light on a timescale of 3.5-5 yr. The optical spectra of these novae display quite different ch aracteristics from each other, with DI Lac showing narrow Balmer emission c ores situated in broad absorption troughs while V841 Oph exhibits strong si ngle-peaked Balmer, He I, and He II emission lines. We find little change b etween spectra obtained during different optical brightness states. The X-r ay count rates for both novae were very low (less than or similar to1.5 cou nts s(-1)), and there was no reliable correlation between X-ray and optical brightness. The combined X-ray spectrum of DI Lac is best fit by a bremsst rahlung emission model (with kT similar to 4 keV and N-H < 1.8 x 10(22) cm( -3)); the X-ray spectrum of V841 Oph is too weak to allow model fitting. We discuss the possible origin of variability in these old novae in terms of magnetic activity on the secondary star, dwarf nova type disk instabilities , and the "hibernation" scenario for cataclysmic variable stars.