Photometry and Spectroscopy of the Optical Companion to the Pulsar PSR J1740−5340 in the Globular Cluster NGC 6397

We present photometric and spectroscopic observations of the optical companion to the millisecond radio pulsar PSR J1740-5340 in the globular cluster NGC 6397. An analysis of the photometric variability in the B, V, and I bands indicates an inclination of the system of 43.°9 ± 2.°1 if the optical companion fills its Roche lobe (a semidetached configuration). The spectroscopic data show a radial velocity variation with a semiamplitude of K = 137.2 ± 2.4 km s-1, and a system velocity γ = 17.6 ± 1.5 km s-1, consistent with cluster membership. We use these results to derive a mass of the optical companion of M1 = 0.296 ± 0.034 M⊙ and M2 = 1.53 ± 0.19 M⊙ for the pulsar. There is evidence for secular change of the amplitude of the optical light curve of the variable measured over 7 years. The change does not have interpretation and its presence complicates reliable determination of the absolute parameters of the binary.