My solution was to mount a resistor 27 Ohm - 50 W in parallel to one of the lampfittings, see picture. With the additional 5 W of this resistor I can use three LED lamps of 2.4 W each. The total power is now approx. 20W (5W resistor, 7W LED, 8 W electronics) and the LEDS can be dimmed down to 5 W.