The hierarchical triple system PSR J0337+1715 offers an unprecedented laboratory to study secular evolution of interacting systems and to explore the complicated mass-transfer history that forms millisecond pulsars and helium-core white dwarfs .
The latter in particular , however , requires knowledge of the properties of the individual components of the system .
Here we present precise optical spectroscopy of the inner companion in the PSR J0337+1715 system .
We confirm it as a hot , low-gravity DA white dwarf with T _ { eff } = 15 , 800 \pm 100 K and \log _ { 10 } ( g ) = 5.82 \pm 0.05 .
We also measure an inner mass ratio of 0.1364 \pm 0.0015 , entirely consistent with that inferred from pulsar timing , and a systemic radial velocity of 29.7 \pm 0.3 { km } { s } ^ { -1 } .
Combined with the mass ( 0.19751 M _ { \odot } ) determined from pulsar timing , our measurement of the surface gravity implies a radius of 0.091 \pm 0.005 R _ { \odot } ; combined further with the effective temperature and extinction , the photometry implies a distance of 1300 \pm 80 pc .
The high temperature of the companion is somewhat puzzling : with current models , it likely requires a recent period of unstable hydrogen burning , and suggests a surprisingly short lifetime for objects at this phase in their evolution .
We discuss the implications of these measurements in the context of understanding the PSR J0337+1715 system , as well as of low-mass white dwarfs in general .