Optical studies of millisecond pulsar companions

In observations of four binary millisecond pulsars with the Palomar 200 inch (5.1 m) telescope, we have detected white dwarf companions to PSR J1640+2224 and PSR J2145 - 0750 and placed upper limits of m(r) = 24 and m(r) = 25 on companions to PSR J0751+1807 and PSR J2229+2643. The temperature inferred for the companion to PSR J1640+2224 is T = 3700 +/- 300 K. The luminosity inferred for a star at a distance of 1.2 kpc is consistent with that expected from a (2 +/- 1) x 10(9) yr old helium white dwarf. For PSR J2145-0750, the measured colors are inconsistent, possibly due to a combination of emission lines in the spectrum and systematic effects introduced when subtracting a nearby star. The companion temperature derived lies in the range 4300- 6800 K. For a carbon-oxygen white dwarf, the corresponding cooling age is 2-6 x 10(9) yr. The cooling ages of both systems with detected white dwarf companions are significantly less than the upper limits from the pulsar spin-down ages. For both pulsars, the inferred accretion rate for mass transfer during Roche lobe overflow is 2 orders of magnitude below Eddington accretion rates.