  Multi-color ($B$ $V$ $R_c$ $I_c$) CCD photometric light curves of the contact
binary V502 Oph are analyzed using the Wilson-Devinney (W-D) program. The
solutions reveal that V502 Oph is a W-subtype contact ($f = 35.3\,\%$) binary
system. The temperature difference between its two components is $240K$ and the
more massive star has a lower surface temperature. A cool spot is added in our
model to account for the light curves' asymmetry (O'Connell effect) and third
light is detected for the first time in the light curves' modeling. Combining
the orbital inclination ($i = 76.4^{\circ}$) with the published mass function
of V502 Oph, the absolute physical parameters of the two components are
determined, which are $M_1= 0.46(\pm0.02)M_\odot, M_2=1.37(\pm0.02)M_\odot,
R_1=0.94(\pm0.01)R_\odot, R_2=1.51(\pm0.01)R_\odot, L_1=1.13(\pm0.02)L_\odot$
and $L_2=2.49(\pm0.03)L_\odot$. The formation and evolutionary status of V502
Oph are discussed. All photoelectric and CCD times of light minimum about V502
Oph are gathered and its orbital period variations are analyzed. The results
show that the orbital period of V502 Oph is decreasing continuously at a rate
of $dP/dt=-1.69\times{10^{-7}}day\cdot year^{-1}$, which corresponds to a
conservative mass transfer rate of
$\frac{dM_{2}}{dt}=-3.01\times{10^{-8}}M_\odot/year$. The light-travel time
effect (LTTE) is due to the presence of a close-in tertiary component with a
period of $P_3=18.7$ years and an amplitute of 0.00402days. V502 Oph is an
ideal target to test the formation and evolution theories of binary and
multiple systems in which the light curves, $O - C$ curve and spectroscopic
observations are comprehensively researched.
