Thank you Jerome and thank you Robert. I am going to try to answer to you both at the same time.
This project is done in a house in Setubal where poets, writers, cartoonists - like
André Carrilho 
for example - are received and expose their work.
I started this project in a room with white walls.
At the beginning of the project I used to photograph using a flash hand holded by someone. But that seemed like no solution. At certain moments, the flash was pointing to the ceiling or floor while some words were exchanged !
So, I bought 2 light stands so I would be able to use two flashes instead of only one. Oh that is good.
The first set of images was done in that white room I mentioned. I was told later on that a "second generation" of portraits would be welcome. There, I thought that I could add a black tissue hanging on a wall and use it as a solid background to change the look of the series. And so I did. This is the reason some images are with light background and others - like the earliest - are in black background.
I like best this solution. The black background can be easily made... with HSS need be.
I also have a gray tissue but it has too many wrinkles.
Regarding the position of the body in the frame I tend to frame much at visual evaluation not caring very much if they are centered or not. However, one thing I don't want is that my portraits look like those made in a professional studio.
Perhaps I have - oh yes I do have - more options to chose from the session with this beautiful Ana Maria.
Thank you Robert for pointing to the hands.
Indeed you make me think that the inclusion of the hands is an important matter.
In this project I have done so but not intentionally like here below for example. The hands contribute very much to the strength of the image. This image below gave me much work to adjust to get some detail from both black and white areas.
