Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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 Lacertae
and V841 Ophiuchi. Our optical photometry data (obtained with the automated
photometry telescope RoboScope) comprise an almost decade-long light curve for
each star, while the contemporaneous spectroscopy and X-ray observations
repeatedly sampled each nova during separate intervals of ~45-55 d in length.
The long-term optical light curves of both novae reveal quasiperiodic
variability on typical time scales of ~30-50 d with amplitudes of dV ~ 0.4-0.8
mag. V841 Oph also displays a long-term, sinusoidal modulation of its optical
light on a time scale of 3.5-5 yr. The optical spectra of these novae display
quite different characteristics from each other, with DI Lac showing narrow
Balmer emission cores situated in broad absorption troughs while V841 Oph
exhibits strong single-peaked Balmer, He I and He II emission lines. We find
little change between spectra obtained during different optical brightness
states. The X-ray count rates for both novae were very low (< ~1.5 ct/s) 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 bremsstrahlung emission
model (with kT ~ 4 keV and N_H < 1.8x10^22 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.

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The categories of this paper are: gr-qc
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The categories of this paper are: hep-ph
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The categories of this paper are: eess
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The categories of this paper are: quant-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: q-alg
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The categories of this paper are: stat
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The categories of this paper are: plasm-ph
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The categories of this paper are: cmp-lg
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The categories of this paper are: nlin
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The categories of this paper are: cs
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The categories of this paper are: hep-th
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The categories of this paper are: math
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The categories of this paper are: nucl-th
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The categories of this paper are: adap-org
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The categories of this paper are: math-ph