We have measured the brightness of the optical counterpart of the black hole X-ray binary Nova Sco 1994 (GRO J1655-40) during 40 nights between 1995 May 3 and July 27. From our observations and data from the literature we refine the orbital period to be 2.620 40+/-0.000 98 d. We model the R-band light curve as primarily arising from an X-ray heated secondary, using a model that includes a Roche lobe filling secondary star, the effects of X-ray heating on both the concave accretion disc and the X-ray illuminated surface of the secondary, shadowing of the secondary and disc, and mutual eclipses of the disc and the secondary star. From the shape of the light curve we constrain the inclination of the system to lie in the range 65 deg-76 deg and constrain the mass ratio q=M_1/M_2 to lie in the range 3.8-5.5. This implies a mass range for the secondary star and compact object of 0.95-1.8 Msolar and 4.9-6.8 Msolar, respectively. The (V-R) and (R-i) colour curves do not show evidence of significant variability with orbital phase during 1995 June-July, their average values being 1.06(3) and 0.86(2) mag, respectively.