  This paper is devoted to the 3-dimensional relative differential geometry of
surfaces. In the Euclidean space $\R{E} ^3 $ we consider a surface $\varPhi
%\colon \vect{x} = \vect{x}(u^1,u^2) $ with position vector field $\vect{x}$,
which is relatively normalized by a relative normalization $\vect{y}% (u^1,u^2)
$. A surface $\varPhi^*% \colon \vect{x}^* = \vect{x}^*(u^1,u^2) $ with
position vector field $\vect{x}^* = \vect{x} + \mu \, \vect{y}$, where $\mu$ is
a real constant, is called a relatively parallel surface to $\varPhi$. Then
$\vect{y}$ is also a relative normalization of $\varPhi^*$. The aim of this
paper is to formulate and prove the relative analogues of two well known
theorems of O.~Bonnet which concern the parallel surfaces (see~\cite{oB1853}).
