Companion star of a binary millisecond pulsar

Binary pulsars offer important clues about the evolution of neutron stars. We have searched for and identified the companion star of the recently discovered¹ binary millisecond pulsar PSR1855 + 09. The optical counterpart is compatible with a 0.3M_solar white dwarf with a surface temperature of ~5,900 K. This is in accord with currently accepted ideas on the genesis of such a fast binary pulsar². The low temperature of the white dwarf implies that the system is >10⁹yr old. Such an old pulsar is not easily accounted for by the conventional theory, in which the pulsar ceases to be active after its magnetic field decays, a process believed to take ~10⁷ yr.