Context : The predicted orbital period histogram of a subdwarf B ( sdB ) population is bimodal with a peak at short ( < 10 days ) and long ( > 250 days ) periods .
Observationally , however , there are many short-period sdB systems known , but only very few long-period sdB binaries are identified .
As these predictions are based on poorly understood binary interaction processes , it is of prime importance to confront the predictions to well constrained observational data .
We therefore initiated a monitoring program to find and characterize long-period sdB stars .

Aims : In this contribution we aim to determine the absolute dimensions of the long-period binary system PG 1104+243 consisting of an sdB and a main-sequence ( MS ) component , and determine its evolution history .

Methods : High-resolution spectroscopy time-series were obtained with HERMES at the Mercator telescope at La Palma , and analyzed to determine the radial velocities of both the sdB and MS components .
Photometry from the literature was used to construct the spectral energy distribution ( SED ) of the binary .
Atmosphere models were used to fit this SED and determine the surface gravity and temperature of both components .
The gravitational redshift provided an independent confirmation of the surface gravity of the sdB component .

Results : An orbital period of 753 \pm 3 d and a mass ratio of q = 0.637 \pm 0.015 were found for PG 1104+243 from the radial velocity curves .
The sdB component has an effective temperature of T _ { eff } = 33500 \pm 1200 K and a surface gravity of \log { g } = 5.84 \pm 0.08 dex , while the cool companion is found to be a G-type star with T _ { eff } = 5930 \pm 160 K and \log { g } = 4.29 \pm 0.05 dex .
When a canonical mass of M _ { sdB } = 0.47 M _ { \odot } is assumed , the MS component has a mass of M _ { MS } = 0.74 \pm 0.07 M _ { \odot } , and its temperature corresponds to what is expected for a terminal age main-sequence star with sub-solar metalicity .

Conclusions : PG 1104+243 is the first long-period sdB binary in which accurate and consistent physical parameters of both components could be determined , and the first sdB binary in which the gravitational redshift is measured .
Furthermore , PG 1104+243 is the first sdB+MS system that shows consistent evidence for being formed through stable Roche-lobe overflow .
An analysis of a larger sample of long-period sdB binaries will allow for the refinement of several essential parameters in the current formation channels .