1 Introduction

Superimposed on the extinction curve are a huge number of absorption lines, the diffuse interstellar bands (DIBs). The identification of the DIB carriers remains an important problem in astronomy. The current number of $\sim$200 DIBs is still increasing, suggesting that more than 400 DIBs down to the confusion limit could be detected in the interstellar medium. The development of DIB research in recent years indicates that most DIB carriers could be large carbon-bearing molecules which reside ubiquitously in the interstellar gas (see Herbig 1995 for a review, Salama et al. 1996). The first detection of substructures in the profile of several DIBs indicated the molecular nature of some DIB carriers (Sarre et al. 1995; Ehrenfreund & Foing 1996). Foing & Ehrenfreund (1994, 1997) observed two DIBs at 9577 and 9632 Å as first evidence for C₆₀⁺, the largest molecule ever detected in space. A survey of DIB correlations over 4000 Å showed that most of the DIB carriers are undergoing photo-ionization and that all measured DIBs do originate from different carriers (Cami et al. 1997).

Recently many laboratories, theoreticians and observational astronomers showed combined efforts to solve the long-standing mystery of the DIBs and to identify their carriers. The spectra of PAH and fullerene cations, measured in a Neon matrix, carbon chains measured in the gas phase and theoretical calculations of the non-linear H₂-DIB model have all shown some coincidences with some diffuse bands (see Salama et al. 1996; Foing & Ehrenfreund 1997; Freivogel et al. 1994; Sorokin et al. 1996; Ubachs et al. 1997; Tulej et al. 1998).

Another approach to identify the DIB carriers is to study the complete DIB spectrum in different interstellar and circumstellar regions and to relate the line-of-sight conditions directly to the formation/evolution and destruction properties of DIB carrier molecules. Additional observations of spectral molecular features such as CH, CH⁺, CN (as well as atomic lines CaI, CaII, NaI) reveal variations of physical parameters of the interstellar environment and can constrain the chemistry, ionization balance, metallicity and electron density in the circumstellar and interstellar environment. To define those parameters and to relate them to the DIB behaviour in the same region allows another view on the nature of the DIB carriers.

Observations show currently up to $\sim$200 DIBs in dense and cold environments as well as in UV dominated regions. Their central wavelength is extremely constant. The band strength of the strongest DIBs (such as the 5780 and 5797 Å DIBs, which are measured towards more than 200 sources) does not change by more than a factor $\sim$2. The relative DIB strength, W/E_(B-V), seems to reflect an interplay between ionization and recombination/destruction of the DIB carrier molecules. Even the high resolution profile of the 6613 Å DIB, which shows a characteristic triple peak, displays only slight changes in different environments.

A new reference target for DIB studies was recently detected, which shows the strongest DIBs ever measured and allows to define the DIBs in several categories which respond in totally different ways to the local environment (Ehrenfreund et al. 1997). We discuss the optical survey towards the star BD+63$^{\circ }$ 1964 which led to the detection of 60 new DIBs. We present the complete DIB spectrum between 3906 and 6812 Å of BD+63$^{\circ }$ 1964 and two other heavily reddened reference targets, HD 183143 and BD+40$^{\circ }$ 4220, and discuss the statistics of the confirmed DIBs.