Laplacian spectral characterization of dumbbell graphs and theta graphs

Let $P_n$ and $C_n$ denote the path and cycle on $n$ vertices respectively. The dumbbell graph, denoted by $D_{p,k,q}$, is the graph obtained from two cycles $C_p$, $C_q$ and a path $P_{k+2}$ by identifying each pendant vertex of $P_{k+2}$ with a vertex of a cycle respectively. The theta graph, denoted by ${\mathrm{\Theta }}_{r,s,t}$, is the graph formed by joining two given vertices via three disjoint paths $P_r$, $P_s$ and $P_t$ respectively. In this paper, we prove that all dumbbell graphs as well as theta graphs are determined by their Laplacian spectra.