  We consider the Klein-Gordon-Schr\"odinger system \begin{align*} i \partial_t
\psi + \Delta \psi & = \phi^2 \psi - \phi \psi \\ (\Box +1)\phi & = -2|\psi|^2
\phi + |\psi|^2 \end{align*} with additional cubic terms and Cauchy data $$
\psi(0) = \psi_0 \in H^s({\mathbb R}^n) \, , \, \phi(0) = \phi_0 \in
H^k({\mathbb R}^n) \, , \, (\partial_t \phi)(0) = \phi_1 \in H^{k-1}({\mathbb
R}^n) $$ in space dimensions $n=2$ and $n=3$ . We prove local existence,
uniqueness and continuous dependence on the data in
Bourgain-Klainerman-Machedon spaces for low regularity data, e.g. for
$s=-\frac{1}{8}$, $k=\frac{3}{8}+\epsilon$ in the case $n= 2$ and $s=0$ ,
$k=\frac{1}{2}+\epsilon$ in the case $n=3$. Global well-posedness in energy
space is also obtained as a special case. Moreover, we show "unconditional"
uniqueness in the space $\psi \in C^0([0,T],H^s) \, , \, \phi \in
C^0([0,T],H^{s+\frac{1}{2}}) \cap C^1([0,T],H^{s-\frac{1}{2}})$, if $s >
\frac{3}{22}$ for $n=2$ and $s > \frac{1}{2}$ for $n=3$.
