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Previous optical and radio observations of the binary millisecond pulsar PSRJ1640+2224 have come to inconsistent conclusions about the identity of itscompanion, with some observations suggesting the companion is a low-masshelium-core (He-core) white dwarf (WD), while others indicate it is most likelya high-mass carbon-oxygen (CO) WD. Binary evolution models predict PSRJ1640+2224 most likely formed in a low-mass X-ray binary (LMXB) based on thepulsar's short spin period and long-period, low-eccentricity orbit, in whichcase its companion should be a He-core WD with mass about $0.35 - 0.39 \,M_\odot$, depending on metallicity. If it is instead a CO WD, that wouldsuggest the system has an unusual formation history. In this paper we presentthe first astrometric parallax measurement for this system from observationsmade with the Very Long Baseline Array (VLBA), from which we determine thedistance to be $1520^{+170}_{-150}\,\mathrm{pc}$. We use this distance and areanalysis of archival optical observations originally taken in 1995 with theWide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST) inorder to measure the WD's mass. We also incorporate improvements incalibration, extinction model, and WD cooling models. We find that the existingobservations are not sufficient to tightly constrain the companion mass, but weconclude the WD mass is $>0.4\,M_\odot$ with $>90\%$ confidence. The limitingfactor in our analysis is the low signal-to-noise ratio of the original HSTobservations.

Reconciling optical and radio observations of the binary millisecond pulsar PSR J1640+2224