Is it true, that we can see stars behind round objects because of somekind of light refraction?
To a degree, yes. You are referring to gravitational deflection of light.
In a homogeneous medium light travels in straight lines. But gravity in effect makes the medium inhomogeneous by curving the geometry of spacetime (http://www.einstein-online.info/spotlights/geometry_force) around massive objects like our Sun. So light can't stay in an exactly straight line as it travels through curved spacetime.
This curvature enables light to bend slightly around massive objects. The more massive the object and the closer light passes to the mass, the more curved spacetime gets and the more the light bends. In the case of the Sun a light ray passing near the surface is bent 1.75 arc-seconds. This seems small, but it means we can see stars that should be just behind the "edge" of the Sun if we block out the Sun's glare (there are telescopes designed to do that).
The bending of light atound massive objects is one of the key experimental data supporting Einatein's theory of general relativity (https://en.wikipedia.org/wiki/Tests_of_general_relativity).