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              <p>A first aim of this paper is to answer, in the case of real ruled surfaces  $X_l$ of base $\C P^1$, $l \geq 2$, to a question of V.A. Rokhlin :the equivariant isotopy class does not suffice to distinguish theconnected components of the space of smooth real algebraic curves of $X_l$.A second aim is to prove that there exists in these surfaces some real schemesrealized by real flexible curves but not by smooth real algebraic curves.These two results of real algebraic geometry are deduced from the followingcomparison theorem : when  $m = l+ 2k$, $k&gt;0$, the discriminants of thesurface $X_m$ are deduced from those of the surface $X_l$ via weightedhomotheties. All these results are obtained thanks to a study of a deformationof ruled surfaces.</p>
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