A gas turbine engine comprising: a casing structure comprising a radially outer casing portion and a radially inner casing portion; and a sensor system for measuring the clearance between a rotary blade and the casing structure, said sensor system comprising: a transmitter for transmitting an electromagnetic signal; a waveguide coupled with the transmitter for guiding the electromagnetic signal radially inwards through the casing structure and for guiding a reflected electromagnetic signal radially outward through the casing structure; and a receiver for receiving the reflected signal, wherein: the waveguide comprises: a radially outer portion mounted in the radially outer casing portion, the radially outer portion having a bore for the passage of the electromagnetic signal; a radially inner portion mounted in or integral with the radially inner casing portion, the radially inner portion having a bore for the passage of the electromagnetic signal, the radially outer portion and the radially inner portion being separate from each other, and the bore in the radially inner portion being open at its radially inner end; and a sealing portion for bridging and sealing against the radially inner and radially outer portions, the sealing portion comprising a deformable portion. a casing structure comprising a radially outer casing portion and a radially inner casing portion; and a sensor system for measuring the clearance between a rotary blade and the casing structure, said sensor system comprising: a transmitter for transmitting an electromagnetic signal; a waveguide coupled with the transmitter for guiding the electromagnetic signal radially inwards through the casing structure and for guiding a reflected electromagnetic signal radially outward through the casing structure; and a receiver for receiving the reflected signal, wherein: the waveguide comprises: a radially outer portion mounted in the radially outer casing portion, the radially outer portion having a bore for the passage of the electromagnetic signal; a radially inner portion mounted in or integral with the radially inner casing portion, the radially inner portion having a bore for the passage of the electromagnetic signal, the radially outer portion and the radially inner portion being separate from each other, and the bore in the radially inner portion being open at its radially inner end; and a sealing portion for bridging and sealing against the radially inner and radially outer portions, the sealing portion comprising a deformable portion. a transmitter for transmitting an electromagnetic signal; a waveguide coupled with the transmitter for guiding the electromagnetic signal radially inwards through the casing structure and for guiding a reflected electromagnetic signal radially outward through the casing structure; and a receiver for receiving the reflected signal, wherein: the waveguide comprises: a radially outer portion mounted in the radially outer casing portion, the radially outer portion having a bore for the passage of the electromagnetic signal; a radially inner portion mounted in or integral with the radially inner casing portion, the radially inner portion having a bore for the passage of the electromagnetic signal, the radially outer portion and the radially inner portion being separate from each other, and the bore in the radially inner portion being open at its radially inner end; and a sealing portion for bridging and sealing against the radially inner and radially outer portions, the sealing portion comprising a deformable portion. a radially outer portion mounted in the radially outer casing portion, the radially outer portion having a bore for the passage of the electromagnetic signal; a radially inner portion mounted in or integral with the radially inner casing portion, the radially inner portion having a bore for the passage of the electromagnetic signal, the radially outer portion and the radially inner portion being separate from each other, and the bore in the radially inner portion being open at its radially inner end; and a sealing portion for bridging and sealing against the radially inner and radially outer portions, the sealing portion comprising a deformable portion.