I was with my girlfriend waiting for the bus to go back to my house, when I noticed what I believed that was a star moving very slowly. I've told my girlfriend and she confirmed that I was not just seeing things, and even pointed me out another light coming from the same south-west to west direction. Both of them were glowing, but one had am ever changing collor that varied from orange to yellow and the other blue to white. At first, I though they where sattelites, but as soon as I told that to my girlfriend the objects had changed their direction. We've spent about 15 minutes watching the event, when the UFO extinguished its lights immediatly after reaching (far on the sky) a position over us.I was amazed by the scene, as it's not the first time I've saw an UFO, but this time it were different. This time it were a rich experience because of its duration and the fact that when we arrived there on that afternoon I've had had felt that I was going to see something and I must have kept an eye on the sky.Porto Alegre was founded aside a huge water mass called Stuary Guaiba, the local of sighting is in the rural area of the city and less than 10Km away from the stuary. I'm almost sure that the UFO flew from there. The local of sight is also a low-populated and dark area, with no trees to disturb the vision of the sky (in fact, it's possible to sight The Milky Way on the sky with ease from there).