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.degrees9 +/- 2.degrees1 if the optical companion fills its Roche lobe (a semidetached configuration). The spectroscopic data show a radial velocity variation with a semi-amplitude of K = 137.2 +/- 2.4 km s(-1), and a system velocity gamma = 17.6 +/- 1.5 km s(-1), consistent with cluster membership. We use these results to derive a mass of the optical companion of M-1 = 0.296 +/- 0.034 M-. and M-2 = 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.