Orbital period study of two contact W UMa-type contact binaries: V551 Dra and CN Hyi

V551 Dra and CN Hyi are both W UMa-type contact binaries. Basing on TESS data, we conduct analyses of their orbital period variations. The results indicate that the O - C curve of V551 Dra shows a long-term upward trend, with $dP/dt=2.80\times 10^{-7}day\cdot yea{r}^{-1}$, which could be attributed to mass transfer from the less massive component to the more massive one. Additionally, a periodic variation with an amplitude of $A=0.000954days$ and a period of $P=0.31years$ was superimposed. The periodic variation in the orbital period may be due to the light-travel time effect caused by a third body. We estimate the minimum mass of the third body to be $M_3=0.48M_{\odot }$. The primary of V551 Dra, although less massive, is oversized and overluminous compared to standard mainsequence stars, suggesting it has evolved through substantial mass loss, while the more massive secondary remains on the main sequence, indicating a stable mass transfer phase. In contrast, The O - C curve of CN Hyi exhibits a long-term decreasing trend, with $dP/dt=-1.09\times 10^{-6}day\cdot yea{r}^{-1}$. Due to the intermediate contact nature of the binary system, this long-term period decrease is likely to be caused by mass transfer from the more massive primary star to the less massive secondary star. The primary of CN Hyi resides on the main sequence, while the oversized and overluminous secondary suggests significant material accretion. These findings suggest differing evolutionary pathways for W-subtype and A-subtype contact binaries. V551 Dra appears to exhibit stable mass transfer dynamics, whereas CN Hyi may potentially experience more dynamic structural changes as it evolves. In addition to these evolutionary considerations, we hypothesize that the period variation of CN Hyi may also be attributed by the light-travel time effect of a third body, given the common occurrence of third bodies in contact systems. Considering the common occurrence of third bodies in contact binary systems, we hypothesize that the period variation of CN Hyi may also be attributed by the light-travel time effect of a third body. Nevertheless, this conclusion awaits further observational confirmation.