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A V light curve of the W UMa contact binary Y Sex is presented in this paper. From the observations, two times of minimum light was determined and from the present times of minimum light and those collected from the references, the change in the orbital period of the system was analyzed with the method of Kalimeris et al. [A&A 282 (1994) 775]. The result reveals that the orbital period of the system oscillates with a cycle of about 50 years and an amplitude of 1.1x10(-6) days. The light curve was analyzed by means of the latest version of the Wilson-Devinney code, which was also used to correct photometric effects on the radial-velocity curve obtained by McLean and Hilditch [MNRAS 203 (1983) 1]. The results suggest that Y Sex is an A-subtype contact binary with a mass ratio of q=0.180(2). In the new photometric solution, a positive value of the third light of the system suggests that the sinusoidal variation of the period could be caused by a third component in the system. The absolute dimensions of Y Sex are found to be: M-1=1.21(18) M-circle dot, M-2=0.22(3) M-circle dot, R-1=1.50(2) R-circle dot, R-2=0.75(1) R-circle dot, L-1=3.00(44) L-circle dot, L-2=0.69(10) L-circle dot, A=2.66(11) R-circle dot. (C) 2003 Elsevier B.V. All rights reserved.