Context :

Aims : We investigate the structure of the O-type binary system HD 48099 by measuring linear polarization that arises due to light scattering process .
High-precison polarimetry provides independent estimates of the orbital parameters and gives important information on the properties of the system .

Methods : Linear polarization measurements of HD 48099 in the B , V and R passbands with the high-precision Dipol-2 polarimeter have been carried out .
The data have been obtained with the 60 cm KVA ( Observatory Roque de los Muchachos , La Palma , Spain ) and T60 ( Haleakala , Hawaii , USA ) remotely controlled telescopes during 31 observing nights .
Polarimetry in the optical wavelengths has been complemented by observations in the X-rays with the SWIFT space observatory .

Results : Optical polarimetry revealed small intrinsic polarization in HD 48099 with \sim 0.1 \% peak to peak variation over the orbital period of 3.08 d. The variability pattern is typical for binary systems , showing strong second harmonic of the orbital period .
We apply our model code for the electron scattering in the circumstellar matter to put constraints on the system geometry .
A good model fit is obtained for scattering of light on a cloud produced by the colliding stellar winds .
The geometry of the cloud , with a broad distribution of scattering particles away from the orbital plane , helps in constraining the ( low ) orbital inclination .
We derive from the polarization data the inclination i = 17 \degr \pm 2 \degr and the longitude of the ascending node \Omega = 82 \degr \pm 1 \degr of the binary orbit .
The available X-ray data provide additional evidence for the existence of the colliding stellar winds in the system .
Another possible source of the polarized light could be scattering from the stellar photospheres .
The models with circumstellar envelopes , or matter confined to the orbital plane , do not provide good constraints on the low inclination , better than i \leq 27 \degr , as is already suggested by the absence of eclipses .

Conclusions :