  We present the results of new photometric and spectroscopic observations of
WN5+O6 binary V444 Cyg and a detailed analysis of extant spectroscopy and
photometry. Using elements of the spectroscopic orbit and assuming e = 0, i =
78{\deg} we determined the masses and orbit sizes of the components of V444 Cyg
M_O6 = 26.4 M_Sun, M_WN5 = 10.7 M_Sun, a_O6 = 10.6 R_Sun, a_WN5 = 26.1 R_Sun.
Based on new and archival light curves and by applying the Hertzsprung method,
we improved the photometrical estimate of secular increase rate of the orbital
period in V444 Cyg dP/dt_ph = 0.119+-0.003 s/yr. From the comparison of the new
and archival radial velocity curves of V444 Cyg we independently derived the
secular orbital period change rate dP/dt_sp = 0.147+-0.032 s/yr, in agreement
with the photometrical dP/dt_ph. The obtained secular increase rate of the
binary orbital period dP/dt and the mean radii of the components enabled us to
estimate the stellar wind mass-loss rate from WR star dM/dt_WN5 =
-(6.0+-0.4)*10^-6 M_Sun/yr.
