\section{Introduction} \label{s.intro}

In short-period binary systems, the proximity of the stars means that
effects arising from their mutual irradiation can become important. In
low-mass binaries, the principal consequence is the so-called
`reflection effect', where an irradiated surface heats up to the point
where it re-radiates the external radiation falling upon it
\citep[see, e.g.,][and references therein]{Wilson:1990aa}. The same
effect will arise in massive-star binaries, but the immense radiative
fluxes in these latter cases mean that radiative forces can also play
a significant role.

\citet{Drechsel:1995aa} argue that radiative forces can appreciably
distort surface geometries in close massive-star binaries. They employ
a formalism which represents these forces as the gradient of a scalar
potential, allowing the surfaces of the components to be represented
by a modified Roche potential. This formalism has proven quite popular
in the literature
\citep[e.g.,][]{Phillips:2002aa,Tsantilas:2006aa,Palate:2013aa}.

In the present paper, we demonstrate that the \citet{Drechsel:1995aa}
formalism neglects re-emission of incident radiation. When taken into
account, this re-emission results in the complete cancellation of the
vertical component of the external radiative force; thus, no surface
distortion is to be expected. However, the remaining horizontal
component of the force can lead to surface flows, and we develop
analytic (Section~\ref{s.analytic}) and numerical
(Section~\ref{s.numerical}) treatments for these flows. In
Section~\ref{s.calc} we apply these treatments to model an O6V+O6V
binary system, and explore the observational consequences of radiatively
driven surface flows. Our findings are summarized in
Section~\ref{s.summary}.

%% Drechsel (1995)
%%   - represent Frad using integral over intensity supplied by irradiating star.
%%   - neglects pressure contribution by re-radiated flux
%%   - parameterizes effect of local irradiation in terms of a delta parameter: F_eff = F_grav*(1 - delta)
%%   - delta has terms from internal effects and from irradiation
%%   - potential of each star modified by radiation in two ways:
%%     - the gravity of the star itself is reduced by (1-delta) (internal delta)
%%     - the gravity of the other star is reduced by (1-delta*) (external delta)
%%   - irradiation calculated by considering companion (and star, for self-eclipse effects) to be triaxial
%%   - deformed surface found by Newton-Raphson iteration: all points fall on the same equipotential Omega_0
%%   - during NR iteration, it seems that delta* is held fixed (p. 727, para on top right)
%%   - essentially: the gradient of the irradiation terms in their effective potential does not give the 
%%                  force due to irradiation
