Context : Massive stars deeply modify their surrounding ISM via their high throughput of ionizing photons and their strong stellar winds .
In this way they may create large expanding structures of neutral gas .

Aims : We study a new large H i shell , labelled GS305+04-26 , and its relationship with the OB association Cen OB1 .

Methods : To carry out this study we have used a multi-wavelenght approach .
We analyze neutral hydrogen ( H i ) line data retrieved from the Leiden-Argentina-Bonn ( LAB ) survey , new spectroscopic optical observations obtained at CASLEO , and make use of proper motion databases available via Internet .

Results : The analysis of the H i data reveals a large expanding structure GS305+04-26 centered at ( l , b ) = ( 305 ^ { \degr } , +4 ^ { \degr } ) in the velocity range from -33 to -17 km s ^ { -1 } .
Based on its central velocity , -26 km s ^ { -1 } , and using standard galactic rotation models , a distance of 2.5 \pm 0.9 kpc is inferred .
This structure , elliptical in shape , has major and minor axis of 440 and 270 pc , respectively .
Its expansion velocity , total gaseous mass , and kinetic energy are \sim 8 km s ^ { -1 } , ( 2.4 \pm 0.5 ) \times 10 ^ { 5 } M _ { \odot } , and ( 1.6 \pm 0.4 ) \times 10 ^ { 50 } erg , respectively .
Several stars of the OB-association Cen OB1 are seen projected onto , and within , the boundaries of GS305+04-26 .
Based on an analysis of proper motions , new members of Cen OB1 are identified .
The mechanical energy injected by these stars could have been the origin of this H i structure .

Conclusions :