  We propose the determination of the CKM matrix element $|V_{ub}|$ by the
measurement of the spectrum of $B \to D_s^+ X_u$, dominated by the spectator
quark model mechanism $\bar{b} \to D_s^{(*)+} \bar{u}$. The interest of
considering $B \to D_s^+X_u$ versus the semileptonic decay is that more than 50
% of the spectrum for $B \to D_s^+ X_u$ occurs above the kinematical limit for
$B \to D_s^+ X_c$, while most of the spectrum $B \to l \nu X_u$ occurs below
the $B \to l \nu X_c$ one. Furthermore, the measure of the hadronic mass $M_X$
is easier in the presence of an identified $D_s$ than when a $\nu$ has been
produced. As a consistency check, we point out that the rate $\bar{b} \to
D_s^{(*)+} \bar{c}$ (including QCD corrections that we present elsewhere) is
consistent with the measured $BR (B \to D_s^{\pm} X)$. Although the hadronic
complications may be more severe in the mode that we propose than in the
semileptonic inclusive decay, the end of the spectrum in $B \to l \nu X_u$ is
not well understood on theoretical grounds. We argue that, in our case, the
excited $D_s^{**}$, decaying into $D K$, do not contribute and, if there is
tagging of the $B$ meson, the other mechanisms to produce a $D_s$ of the right
sign are presumably small, of $O(10^{-2})$ relative to the spectator amplitude,
or can be controlled by kinematical cuts. In the absence of tagging, other
hadronic backgrounds deserve careful study. We present a feasability study with
the BaBar detector.
