It seems like Crotalaria retusa.

Thanks Arvind but definitely not C. retusa... This species has fruits that are covered with whitish silky hairs while C. retusa has smooth and more slender fruits, and more common in our area... See picture below for C. retusa

Please have a look at this link.

http://www.westafricanplants.senckenberg.de/root/index.php?page_id=14&id=460

Based on flower colour, it seems a Crotalaria pumila...please check this link. 

http://www.fireflyforest.com/flowers/847/crotalaria-pumila-low-rattlebox/

Felix, please contact my colleague from Federal University of Rio Grande do Sul, botany Silvia Mioto, expert in this family, following your email: [email protected]

hug

Ricardo

Dear Félix

It looks like C. incana or C. micans.

Kind regards.

Yannis. 

Thanks guys... I so much appreciate all your suggestions, but there are still doubts as the features of this species seem not to agree with the suggested species and the descriptions given in our local Flora...

Thanks Ricardo, I had just forwarded a mail to your colleague! I hope we get answers! :-)