Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Optical spectroscopy of (candidate) ultra-compact X-ray binaries:
  constraints on the composition of the donor stars   We present optical spectroscopy of several (candidate) ultra-compact X-ray
binaries (UCXBs) obtained with the ESO VLT and Gemini-North telescopes. In only
one of five observed UCXB candidates did we find evidence for H in its spectrum
(4U 1556-60). For XB 1905+00 the optical counterpart is not detected. For the
known UCXBs 4U 1626-67 and XB 1916-05 we find spectra consistent with a C/O and
a He/N accretion disc respectively, the latter is the first optical spectrum of
a He-rich donor in an UCXB. Interestingly, the C/O spectrum of 4U 1626-67 shows
both similarities as well as marked differences from the optical C/O spectrum
of 4U 0614+09. We obtained phase resolved spectroscopy of 4U 0614+09 and the 44
min transient XTE J0929-314. In neither object were we able to detect clear
orbital periodicities, highlighting the difficulties of period determinations
in UCXBs. We reanalysed the spectra of XTE J0929-314 that were taken close to
the peak of its 2003 X-ray outburst and do not confirm the detection of Halpha
emission as was claimed in the literature. The peak spectra do show strong C or
N emission around 4640A, as has also been detected in other UCXBs. We discuss
the implications of our findings for our understanding of the formation of
UCXBs and the Galactic population of UCXBs. At the moment all studied systems
are consistent with having white dwarf donors, the majority being C/O rich.

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The categories of this paper are: supr-con
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The categories of this paper are: mtrl-th
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The categories of this paper are: hep-ex
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The categories of this paper are: gr-qc
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The categories of this paper are: dg-ga
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The categories of this paper are: solv-int
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The categories of this paper are: adap-org
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The categories of this paper are: acc-phys
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The categories of this paper are: nucl-th
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The categories of this paper are: quant-ph
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The categories of this paper are: nlin
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The categories of this paper are: chem-ph
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The categories of this paper are: math-ph
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The categories of this paper are: math
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The categories of this paper are: bayes-an
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The categories of this paper are: q-bio