Context : Long soft gamma ray bursts ( LGRBs ) are usually associated with the death of the most massive stars .
A large amount of core angular momentum in the phases preceding the explosion is required to form LGRBs .
A very high initial rotational velocity can provide this angular momentum .
Such a velocity strongly influences the way the star evolves : it is chemically homogeneously mixed and evolves directly towards the blue part of the HR diagram from the main sequence .

Aims : We have shown that chemically homogeneous evolution takes place in the SMC , at low metallicity .
We want to see if there is a metallicity threshold above which such an evolution does not exist .

Methods : We perform a spectroscopic analysis of H-rich early-type WN stars in the LMC and the Galaxy .
We use the code CMFGEN to determine the fundamental properties ( T _ { eff } , L ) and the surface composition of the target stars .
We then place the stars in the HR diagram and determine their evolution .

Results : We show that both the LMC and Galactic WNh stars we selected can not be explained by standard stellar evolution .
They are located on the left of the main sequence but show surface abundances typical of CN equilibrium .
In addition , they still contain a large amount of hydrogen .
They are thus core-H burning objects .
Their properties are consistent with chemically homogeneous evolution .
We determine the metallicity of the Galactic stars from their position and Galactic metallicity gradients , and conclude that they have 0.6 < Z < 1.0 .
A moderate coupling between the core and the envelope is required to explain that stellar winds do not extract to much angular momentum to prevent a blueward evolution .

Conclusions : We have shown that chemically homogeneous evolution takes place in environments with metallicity up to solar .
In view of the findings that some long gamma ray bursts appear in ( super- ) solar environments , such an evolution may be a viable way to form them over a wide range of metallicities .