  It is an open problem whether Yao-Yao graphs $\mathsf{YY}_k$ (also known as
sparse-Yao graphs) are all spanners when the integer parameter $k$ is large
enough. In this paper we show that, for any integer $k\geq 42$, the Yao-Yao
graph $\mathsf{YY}_{2k}$ is a $t_k$-spanner, with stretch factor
$t_k=6.03+O(k^{-1})$ when $k$ tends to infinity. Our result generalizes the
best known result which asserts that all $\mathsf{YY}_{6k}$ are spanners for
$k$ large enough [Bauer and Damian, SODA'13]. Our proof is also somewhat
simpler.
