Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Optical Studies of the Transient Dipping X-ray Sources X1755-338 and
  X1658-298 in Quiescence   We have studied the optical counterparts of X1755-338 and X1658-298 in their
X-ray "off" states. The first observations of X1755-338 in quiescence show that
the counterpart, V4134 Sgr, has faded by more than 3.5 mag in V. If the mass
donor in the system is an M0V star as implied by the period, our upper limits
on the brightness of the counterpart suggest that it is more distant than 4
kpc. We observed V2134 Oph, the optical counterpart of X1658-298, on several
occasions in April/May 1997 and found the source only about 1 mag fainter than
when it is X-ray bright. Contemporaneous X-ray data confirm that the source
remains in the quiescent state during our optical observations. Our optical
lightcurve, folded on the 7.1 hour orbital period, does not show any modulation
across the binary cycle. It is possible that the absence of detectable X-ray
emission, despite the indication for an accretion disk and activity in the
system, is related to structure in the disk that permanently obscures the
central X-ray source. The optical properties of V2134 Oph are unique among the
known X-ray transients.

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The categories of this paper are: cond-mat
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The categories of this paper are: chao-dyn
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The categories of this paper are: ao-sci
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The categories of this paper are: comp-gas
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The categories of this paper are: math-ph
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The categories of this paper are: supr-con
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The categories of this paper are: cs
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The categories of this paper are: stat
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The categories of this paper are: q-fin
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The categories of this paper are: q-alg
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The categories of this paper are: hep-th
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The categories of this paper are: hep-ex
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The categories of this paper are: math
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The categories of this paper are: cmp-lg
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The categories of this paper are: nucl-ex
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The categories of this paper are: q-bio