  It is known that when hard spheres are added to a pure system of hard rods
the stability of the smectic phase may be greatly enhanced, and that this
effect can be rationalised in terms of depletion forces. In the present paper
we first study the effect of orientational order on depletion forces in this
particular binary system, comparing our results with those obtained adopting
the usual approximation of considering the rods parallel and their orientations
frozen. We consider mixtures with rods of different aspect ratios and spheres
of different diameters, and we treat them within Onsager theory. Our results
indicate that depletion effects, and consequently smectic stability, decrease
significantly as a result of orientational disorder in the smectic phase when
compared with corresponding data based on the frozen--orientation
approximation. These results are discussed in terms of the $\tau$ parameter,
which has been proposed as a convenient measure of depletion strength. We
present closed expressions for $\tau$, and show that it is intimately connected
with the depletion potential. We then analyse the effect of particle geometry
by comparing results pertaining to systems of parallel rods of different shapes
(spherocylinders, cylinders and parallelepipeds). We finally provide results
based on the Zwanzig approximation of a Fundamental--Measure
density--functional theory applied to mixtures of parallelepipeds and cubes of
different sizes. In this case, we show that the $\tau$ parameter exhibits a
linear asymptotic behaviour in the limit of large values of the hard--rod
aspect ratio, in conformity with Onsager theory, as well as in the limit of
large values of the ratio of rod breadth to cube side length, $d$, in contrast
to Onsager approximation, which predicts $\tau\sim d^3$.
