In this paper, we consider the following semilinear elliptic systems: $$\begin{aligned} \left\{ \begin{array}{ll} \displaystyle -\Delta u+V(x)u=F_{u}(x, u, v),\quad \text{ in } \mathbb {R}^{N},\\ -\Delta v+V(x)v=F_{v}(x, u, v),\quad \text{ in } \mathbb {R}^{N},\\ \end{array} \right. \end{aligned}$$ where $$V:\mathbb {R}^{N}\rightarrow \mathbb {R},~F_{u}(x,u,v)$$ and $$F_{v}(x,u,v)$$ are periodic in x. We assume that 0 is a right boundary point of the essential spectrum of $$-\triangle +V$$ . Under appropriate assumptions on $$F_{u}(x, u, v)$$ and $$F_{v}(x, u, v)$$ , we prove the above system has a ground-state solution by using the Nehari-type technique in a strongly indefinite setting. Furthermore, the existence of infinitely many geometrically distinct solutions is obtained via variational methods. Recent results from the literature are improved and extended.