THANKS!
The voltage has now risen to 7.5V. I am not sure whether the motor might be causing such an important voltage drop (1.5V). I will keep investigating.
Thank you very much !
update: removing the dc motor causes the voltage to rise up to 8V. Measuring the battery voltage yields 9.45V.
update 2: using two 4.7k resistors causes the voltage to rise up to 8.5V. Still getting a 1V voltage drop when connecting the motor.
Any clues?