  For an irreducible complex character $\chi$ of the finite group $G$, let
$\pi(\chi)$ denote the set of prime divisors of the degree $\chi(1)$ of $\chi$.
Denote then by $\rho(G)$ the union of all the sets $\pi(\chi)$ and by
$\sigma(G)$ the largest value of $|\pi(\chi)|$, as $\chi$ runs in
${\rm{Irr}}(G)$. The $\rho$-$\sigma$ conjecture, formulated by Bertram Huppert
in the 80's, predicts that $|\rho(G)|\leq 3\sigma(G)$ always holds, whereas
$|\rho(G)|\leq 2\sigma(G)$ holds if $G$ is solvable; moreover, O. Manz and T.R.
Wolf proposed a "strengthened" form of the conjecture in the general case,
asking whether $|\rho(G)|\leq 2\sigma(G)+1$ is true for every finite group $G$.
In this paper we study the strengthened $\rho$-$\sigma$ conjecture for the
class of finite groups having a trivial Fitting subgroup: in this context, we
prove that the conjecture is true provided $\sigma(G)\leq 5$, but it is false
in general if $\sigma(G)\geq 6$. Instead, we establish that $|\rho(G)|\leq
3\sigma(G)-4$ holds for every finite group with a trivial Fitting subgroup and
with $\sigma(G)\geq 6$ (this being the right, best possible bound). Also, we
improve the up-to-date best bound for the solvable case, showing that we have
$|\rho(G)|\leq 3\sigma(G)$ whenever $G$ belongs to one particular class
including all the finite solvable groups.
