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It works this way:

Collect all activities of a teacher and create a "activities occupy min time slots from selection" constraint. "Selection" ist all 1st time slots of the days, and "Min occupied" is 5 (i. e. no. of days a week).
Generating a timetable will probably fail, e. g. because the teacher's "max days per week" is 4.
Just reduce "Min occupied" step by step until I get a feasible solution.

Thanks for the hint!

Regards,
  Friedhelm.

Tried it - works perfectly.
And the size of my .fet file is reduced by 20K  .

Thanks a lot!

(I'll also switch to the newest FET version now...)

Regards,
  Friedhelm.

Okay, I'll try that...

That was quick! 

I must admit I didn't understand the concept of categories yet. I'm going to try harder...

About solution 1):
Activities are then still assigned to the groups, not the subgroups, correct?
Of course, we have more than 4 students, but if I understand correctly, I just need (up to) 8 dummy students (resp. subgroups) "A" to "H", and add them to the groups accordingly?

By the way: I noticed that the "Add existing" Button is only available in the latest version of FET. As I generate the .fet file automatically, I'm a bit careful with migrating to new versions. Did you change anything with the xml-format of the .fet file between versions 5.x and 6.x?

Some teachers are early birds and prefer the 1st time slot of the day.
I'd like to allow that -- as often as possible.

It will not always be possible due to conflicts with other constraints (rooms / students not available etc.). Therefore, a "teacher's not available times" constraint (allowing e.g. only the morning slots) will be too restrictive and lead to impossible solutions.

What's the best way to achieve this?

At our university, we have:
o lectures, where all students of a semester attend, and
o practicals (e. g.) where students of a semester are divided into smaller groups. Sometimes 2 groups, sometimes 4 groups etc.

So I created a "Year" (e. g. SEM1) for each semester, and acoording groups:
SEM1-2.1, SEM1.2.2
SEM1-3.1, SEM1-3.2, SEM1- 3.3
SEM1-4.1 ... SEM1-4.4
... and so on, up to SEM1-8.1 ... SEM1-8.8

Now, suppose we have 4 students: Alice, Bob, Chris and Donald. Students are assigned to groups alphabetically, i. e.:
Alice + Bob are in group SEM1-2.1
Chris and Donald are in group SEM1-2.2
But also:
Alice is in group 4.1 (and also 3.1, 5.1, ...)
Bob is group 4.2
Chris is in group 4.3
Donald is in group 4.4.

From FET's point of view, these groups are all independent. Therefore, it might, e. g., place a "SEM1-2.1"-Activity in parallel with a "SEM1.4.1"-Activity. But that's not possible, because Alice cannot attend both Atctivities at the same time.

To avoid this, I create a lot of "Activities not overlapping" constraints:
o Collect all "SEM1-2.1" Activities plus all "SEM1.4.1" Activities and create a "not overlapping" constraint.
o Collect all "SEM1-2.1" Activities plus all "SEM1.4.2" Activities and create a "not overlapping" constraint.
o ... and so on, with all possible combinations.

Anyway, it's working fine this way. Timetable generation is a matter of minutes or even seconds.
However, I'm just wondering: Is there a more elegant way to do this?