\feynmandiagram[inline=(a.base), horizontal=a to b] {
a[dot,label=left:$V_0$] -- [double charged boson, half right, bend right, edge label'=$b$] b[dot,label=right:$V_0^*$],
a -- [double fermion, edge label'=$c_\sigma$] b -- [double charged scalar, half right, bend right, edge label=$f_\sigma$] a
};