  Let $E$ be a number field and $X$ a smooth geometrically connected variety
defined over a characteristic $p$ finite field. Given an $n$-dimensional pure
$E$-compatible system of semisimple $\lambda$-adic representations of the
\'etale fundamental group of $X$ with connected algebraic monodromy groups
$G_\lambda$, we construct a common $E$-form $G$ of all the groups $G_\lambda$
and in the absolutely irreducible case, a common $E$-form
$G\hookrightarrow\text{GL}_{n,E}$ of all the tautological representations
$G_\lambda\hookrightarrow\text{GL}_{n,E_\lambda}$ (Theorem 1.1). Analogous
rationality results in characteristic $p$ assuming the existence of crystalline
companions in $\text{F-Isoc}^{\dagger}(X)\otimes E_{v}$ for all $v|p$ (Theorem
1.5) and in characteristic zero assuming ordinariness (Theorem 1.6) are also
obtained. Applications include a construction of $G$-compatible system from
some $\text{GL}_n$-compatible system and some results predicted by the
Mumford-Tate conjecture.
