EXCELLENT! Thank you, Pickelmaxime, for sharing!
I mention these guys on my page about the Hirtzstein the mountain, south of the HWK. ( http://pierreswesternfront.punt.nl/index.php?r=1&id=534577&tbl_archief=0#534577" onclick="window.open(this.href);return false; ) These guys were already involved in the first X-mas Battles for the Hartmannswillerkopf in the period 25 December 1914 - 19 January 1915.
Not to be confused with the somewhat more famous Xmas Battles of the HWK of one year later, the so called "Weihnachtskaempfe", which started at 21 December 1915.
In april this year I will be giving a lecture amongst others about these HWK X-mas battles. So, Maxime, you will probably understand well, why I find this rare picture of U.R. 15 so interesting and why I appreciate it that you share this extra ordinary photo with us!
http://pierreswesternfront.punt.nl/upload/Guebwiller/periodsketch-28_12_1914.jpg