Printer for printing labels, in particular in the form of endless label paper, comprising: - a holder for a label reel having the endless label paper , - a printing unit with a print roller and a print head , wherein the endless label paper can be conveyed along a paper path from the label reel towards the printing unit , wherein the paper path is formed by at least one deflecting pulley , - a cutting device which is arranged along the paper path , - a detector unit for detecting markings which are applied at regular intervals to the endless label paper , wherein the detector unit comprises a first transmitter which outputs light in the UV wavelength range and comprises an associated receiver which receives light in the visible wavelength range, wherein the endless label paper to be printed is an endless strip which is designed without a carrier strip and from which individual labels can be severed, wherein the endless label paper has a printable front-side strip surface and a rear-side strip surface opposite the latter, characterized in that the rear-side strip surface has an adhesive layer , wherein the endless label paper is designed in such a manner that self-adhesive labels can be severed at cutting lines transversely with respect to the longitudinal direction of the endless label paper , wherein the cutting lines are in each case surrounded on the strip rear side by a region which is free from an adhesive layer, wherein the endless label paper has an optical brightener substance which is integrated therein or is supplied thereon and which emits incident light in the UV wavelength range at least partially as light in the visible wavelength range, wherein the markings to be detected by the detector indicate the positions of the regions free from an adhesive layer, wherein the markings to be detected cover the optical brightener substance such that the at least partial emission of the incident light in the UV wavelength range as light in the visible wavelength range is at least partially suppressed in the region of the markings , and wherein the markings are very substantially invisible in the visible wavelength range, in particular do not differ, or do not substantially differ, from regions without marking of the endless label paper in the wavelength range of the visible light, and wherein the printer comprises a control unit which evaluates the receiver signal of the receiver of the detector unit , wherein the receiver receives dark interruptions of a light signal while the endless label paper is conveyed along the detector unit , and the control unit detects a marking on the endless label paper at each dark interruption of the received light. - a holder for a label reel having the endless label paper , - a printing unit with a print roller and a print head , wherein the endless label paper can be conveyed along a paper path from the label reel towards the printing unit , wherein the paper path is formed by at least one deflecting pulley , - a cutting device which is arranged along the paper path , - a detector unit for detecting markings which are applied at regular intervals to the endless label paper , wherein the detector unit comprises a first transmitter which outputs light in the UV wavelength range and comprises an associated receiver which receives light in the visible wavelength range, wherein the endless label paper to be printed is an endless strip which is designed without a carrier strip and from which individual labels can be severed, wherein the endless label paper has a printable front-side strip surface and a rear-side strip surface opposite the latter, <EOS>