  We present the first X-ray study of Collinder 261 (Cr261), which at an age of
7 Gyr is one of the oldest open clusters known in the Galaxy. Our observation
with the Chandra X-ray Observatory is aimed at uncovering the close interacting
binaries in Cr261, and reaches a limiting X-ray luminosity of Lx ~ 4e29 erg/s
(0.3-7 keV) for stars in the cluster. We detect 107 sources within the cluster
half-mass radius r_h, and we estimate that among the sources with Lx >~ 1e30
erg/s, ~26 are associated with the cluster. We identify a mix of active
binaries and candidate active binaries, candidate cataclysmic variables, and
stars that have "straggled" from the main locus of Cr261 in the
colour-magnitude diagram. Based on a deep optical source catalogue of the
field, we estimate that Cr261 has an approximate mass of 6500 M_sun, roughly
the same as the old open cluster NGC6791. The X-ray emissivity of Cr261 is
similar to that of other old open clusters, supporting the trend that they are
more luminous in X-rays per unit mass than old populations of higher (globular
clusters) and lower (the local neighbourhood) stellar density. This implies
that the dynamical destruction of binaries in the densest environments is not
solely responsible for the observed differences in X-ray emissivity.
